I noticed the group doesn't have any normal ways of joining without an invite so here's a tutorial on how to fix that.
How to Manage a GroupAs for why I left as an Admin...well I'm an admin of to many groups as it is and it makes it when someone wants to invite me as an admin of another group rather annoying so I decided to lessen the amount of groups I'm apart of as an admin.
Might help to make it so people can join as members rather than admins.