How to rejoin a group chat
If you want to rejoin a group chat you left, the group admin will need to re-invite you to the group. If you were the only group admin and left the group, another participant was chosen at random to become the new admin. You can ask the admin to re-invite you to the group and make you a group admin again.
Rejoin a group you've left twice
If you want to rejoin a group you’ve left twice, you must wait 24 hours before the group admin can re-invite you. To avoid the wait time to re-invite a participant, the admin can share a group invite link with you.
Note: Every time you leave a group, the amount of time you must wait to be re-added by the group admin exponentially increases. The maximum amount of time you’d have to wait would be 81 days.