A lot of authors can write some truly deplorable things,
Challenge accepted. *knuckle crack

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Anyway,
I don't trust myself to be in a group because I'm me,
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-but As
@RivCA said: schedules are a huge must.
I would start off by checking your own schedule
@Blackout , to see what days you would be available, (seeing if your days match up with anyone else's on SH.) and then adding more information to the post that will shorten the list of people that are browsing by. Just for example: You wouldn't want someone coming by saying they're free every monday, when that's your most busiest day of every week. you know? then ask yourself what you're really looking for or expecting from the collab.
will it be a here (on SH), Or strictly a discord thing?
will it be written out completely then posted, or posted weekly, biweekly, monthly? or posted once the quota of words or pages are filled?
who will be posting it? will there be a shared SH account everyone in the group has the password to? (incase someone drops out and decides they dont want to be the one writing it anymore. or maybe theres a fight between members)
who gets credit for what?
how many people are you looking for? or will be allowed up to? is there an age range?
what kind of genre are you leaning toward? are there any themes you want to stay away from?
are there limitations to ideas?
will there be rules against using the shared universe created together and using it in individuals stories? like spinoffs?
will this just be a one time thing? or will there be multiple books or series if its popular enough?
I think group efforts need a lead. Are you willing to take that lead, and make sure everyone is doing their part or showing up? can people be kicked out? and if so, will their ideas still be allowed to be used, or if they say no, and you're willing to do them that favor, will you be able to just drop that portion even if it is integral to the story, or ignore them?
I don't want to dishearten the idea. I've always wanted to do a writing collab, but like i said, i just don't trust myself enough to do it. You on the other hand were willing to ask, and have experience in it. I just wouldn't want to see anyone get hurt (including yourself, again) by the idea if/when it becomes reality, so I'm throwing you my thought process of a few things you should probably consider beforehand.
Overall, If/when it comes to fruition, I hope its a fun and exciting experience for everyone involved! I wish you luck!