First band I was in was a band covering a couple of Go Gos songs as part of our Prom Aid festival to raise money for our high school prom. I don’t think the OP is interested in cover bands, so moving on…
Next bands I joined were just small groups from for the Introduction to Music Making course I took in university. The course should have been entitled Introduction to Experimental Music Making, because more than half the students joined thinking we were all going to just whistle and play castanets to nursery rhymes, or do the hippie drum circle… just something easy. Some fled in horror after the professor asked us how we discover music, what role various marketing campaigns went into said “discovery” of music, etc. - some kids just don’t want to be in a class where you’re going to be forced to actually think and ponder with your own brain. The others who left realized that we were going to do something harder than just castanets and nursery rhymes - we were going to be asked to at least attempt making art out of sound.
First rock band I joined was a punk band that my roommate at the time was putting together. Well, it was really more of an audition than being asked to be a full band member. The roommate regularly went to shows at The Casbah, mostly punk shows but it was through that scene that he got into Henry Threadgill, Anthony Braxton, etc. On the punk side, he was into Slint and bands local to our city like Rocket from the Crypt, Drive like Jehu, etc. I’d never though of the intersection of punk, free jazz and some flavors of prog rock (esp. King Crimson and Faust) before, but that was where these guys were at. After some band rehearsals, I was informed I failed the audition, but we still remained friends and roommates.
I didn’t join any more bands until I moved to the DC area. I got bored one day and look at Craigslist and answered an ad put up by a guitarist who wanted to play music like Mogwai, Explosions from the Sky, and Mono. So I joined as a bassist, later asked to switch to 2nd guitar so a new bandmember could play bass. It helped that the founding guitarist already had songs prepared. I had no idea for songs and neither did the drummer, so we just had to create our parts for the songs. Being asked to switch to guitar got me started on spending money on gear again. In short order, I went from having 0 guitars to 3 in just a few months, got a tube amp, pedals, etc. Anyway, for the first few months, we met up and jammed in the basement like clockwork. Eventually though, the jams started to get shorter as we started to spend more time just hanging out and drinking. Towards the end, we’d meet up and not play at all - just drink and pass weed around. We did play one show with an audience. Then shortly after the founding guitarist moved out of town and that was the end of the band.
The bassist of the band asked me to help her out with her music, after she had gained confidence in her guitar playing and started writing a bunch of songs with guitar. So over the next few years, I was in and out of bands led by her. We’re pretty good friends now but COVID-19 has halted our band activities for the time being, because we live in different cities now. I know some people use streaming tech of various sorts to collaborate online but that’s not her thing.
I really don’t have any pearls of wisdom to offer. I haven’t formed any bands of my own yet. Every band I’ve been in, post-university, has been driven by someone else’s songwriting. As a contributor to someone else’s songs, I’ve gotten the habit of being someone conservative - I don’t add fancy superimposed harmonies or syncopated rhythms. The one area I’m given license to be wild and free is effects and foley-style noise.