How to rejoin a group chat
To rejoin a group chat you’ve left, you’ll need to ask the group admin to re-invite you. In some cases, if the group admin has allowed them to in settings, other group participants can re-invite you.
If you were the only group admin and left the group, another participant was chosen at random to become the new admin.
Rejoin a group you've left twice
If you want to rejoin a group you’ve left twice, you’ll need to wait 24 hours before the group admin or other participants can re-invite you.
To avoid the wait time to re-invite a participant, the admin or another participant 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 increases. The maximum amount of time you’d have to wait would be 81 days.
How to change group admin settings