Doesn't really matter much, anyway; Wells included "talpo" with the metaphorical meaning in an old edition of the dictionary, and that word largely didn't survive.

You can use these silly words as much as you like; only time will tell which will survive and which won't. I don't know anyone who still calls anything "groovy". Who knows if the word "cool" will even survive?

How about "kuniĝi"? Technically it has meanings like "come together, associate" and whatnot but in the context of "ĉu vi volas kuniĝi kun mi kaj miaj amikoj morgaŭmatene?" it seems rather clear what the meaning is (to me at least).

Talpo in the metaphorical sense might make a come back with a new era of Russian espionage. "Groovy" may have gone out with 45s and LPs, but as mojosa is in the latest edition of Wells, I predict that it will be around for much longer.

I'm not holding my breath one way or the other. The metaphorical sense of "talpo" was in the "current" edition of Wells from the past, and it didn't really survive.

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I would never say "kuniĝi" in the sense of "to hang out". I would translate it as "to get together", which is different. Hanging out, I would translate as maybe "kunumi". -um- is useful for these vague idiomatic expressions.

picumi = go out for pizza together
kafejumi = hang out in a cafe
glaciaĵumi = go out for ice cream together

Is not "getting together" the same as "hang out"? Last time I checked "hanging out" literally meant multiple people coming to the same location to be together, with the insinuation of staying in such a state assumed by the listener.

Kunumi sounds nice for the idea of "hang out" but I wouldn't say it's better than "kuniĝi". If anything, outside of "kun-" I would find it hard to understand what's being said with things like "picumi" etc, and would automatically assume that the person was trying to say "fari picon" in a shorter form.

Well, at least that's the general curse for "-um-", because it could mean (virtually) anything, one expects it to be used when someone needs a "wild card" to get them out of trouble (sorry for the Uno terminology ).

erinja:Hanging out, I would translate as maybe "kunumi". -um- is useful for these vague idiomatic expressions.

picumi = go out for pizza together
kafejumi = hang out in a cafe
glaciaĵumi = go out for ice cream together

I really like this usage of "-um" ever since I've read about it in one of Piron's articles, and I've been using these since then.

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Neologisms are created out of necessity within a historical context, and as long as necessity and (but not always) the context remains, the neologisms remain in usage. The case of "talpo" shows that. After I read that it appeared in the 80's, I immediately felt that this word (or rather, its metaphorical meaning) came out within the context of the birth of Raumism and its manifesto, which certainly regarded Esperanto propaganda with disdain. And twenty years after, the meaning no longer has relevance in the age of the Internet, and so it fell out of use. [Well, I could be wrong about this, certainly. It's all conjuncture.]

So, in my opinion, the question of whether the neologism "mojosa" will survive rests upon its relevance for future Esperantists. And I doubt that there will be a time when "coolness" will become irrelevant. [But who knows? ]

erinja:picumi = go out for pizza together
kafejumi = hang out in a cafe
glaciaĵumi = go out for ice cream together

They're okay, without the "together" though

picumi definitely wouldn't mean "fari picon", otherwise you'd be saying "fari picon". The whole point of using -um- is that you're expressing something that would otherwise require a whole bunch of words.

Coming together does not necessarily mean hanging out. At all. Hanging out means getting together informally to have a fun time with your friends. Kuniĝi is fully neutral on meaning, it only means to be together with another person. If I am at work, and I go to my colleague John's desk, and I sit down to discuss a business matter with him - we have met the requirements of "kuniĝi" (to get together), but we are not by any means hanging out. We are doing our jobs, not having a fun time. John and I might not even be friends. I might even hate his guts and never want to hang out with him in a million years, but I am only sitting and talking to him because this is work and we are professionals.
[hypothetical situation, I don't happen to work with anyone named John]

erinja:picumi definitely wouldn't mean "fari picon", otherwise you'd be saying "fari picon". The whole point of using -um- is that you're expressing something that would otherwise require a whole bunch of words.

Well, kunmanĝi pico(j)n is just two words, and it is a fair and approximate enough translation of what is commonly meant by "picumi". I agree that in theory it could be understood differently, as, for example, "to cover someone, or to be covered, with pizzas", or "to create a pizza out of nothing by scrupolously following every necessary step", or "to disguise oneself as a pizza for a halloween party"

Kuniĝi is fully neutral on meaning, it only means to be together with another person.

It can actually refer to objects as well, not necessarily to persons.

erinja:picumi definitely wouldn't mean "fari picon", otherwise you'd be saying "fari picon". The whole point of using -um- is that you're expressing something that would otherwise require a whole bunch of words.

The point being that "umi" has no real meaning, it's more a "placeholder". A learner might feel that "fari picon" for whatever reason is not good enough and might try to use "umi". "Ni picumu!" might be a great phrase when people have decided to get pizza, but without the additional context it can mean anything pizza related. Making pizza, going to get pizza, generate a pizza out of pure psychic energy, turning someone into a pizza, etc (of course the latter is a bit silly when you've both iĝi and igi).

Coming together does not necessarily mean hanging out. At all. Hanging out means getting together informally to have a fun time with your friends. Kuniĝi is fully neutral on meaning, it only means to be together with another person.

Then "kunumi" is no better. Ultimately, it depends on how you want to look at it. To me, "coming together" and "getting together" both can be used for "hanging out". In fact, I barely ever "come together" with someone I don't plan on "hanging out" with, and indeed even if it is to do with an assignment or something mundane, "hanging out" isn't necessarily barred from use.

If I am at work, and I go to my colleague John's desk, and I sit down to discuss a business matter with him - we have met the requirements of "kuniĝi" (to get together), but we are not by any means hanging out. We are doing our jobs, not having a fun time. John and I might not even be friends. I might even hate his guts and never want to hang out with him in a million years, but I am only sitting and talking to him because this is work and we are professionals.
[hypothetical situation, I don't happen to work with anyone named John]

At the same time, "kunumi" might mean the same thing. In fact, "hanging out" doesn't even warrant its friendly or aimless connotations, because it's (ah forgot the word, figurative or something like that).

If you were to "kunumi" with John, you are essentially meeting/being with/etc him the same as if you "kuniĝi".

If you want to make a distinction, then context is needed. After all, you don't just ask someone to meet up without any context in English, and the only reason we use "hang out" without context at times is because it's an ingrained habit.

"Hey Jerry, wanna hang out this friday? I need some help with my thesis".
"Hey Jerry, wanna meet up at the mall this friday? We need to catch up soon before I go on my cruise to Boggaboggadongaloo".

"Sal' Ĝeri, ĉu vi volas kuniĝi/kunumi kun mi vrendrede? Mi bezonas helpon kun mia tezo"
"Sal' Ĝeri, ĉu vi volas kuniĝi/kunumi ĉe la 'centro? Estus bonege se ni povus paroli pri la nunaferoj antaŭ ol mia vojaĝo al Bogabogadongaluo"

Doesn't seem problematic to me at all.
Maybe it's a personal distinction, but I believe it's impossible to truly translate the sense of hanging out from English to Esperanto. To emulate it with kuniĝi or your kunumi is possible, but with limitations.