Rejects requests to join a group.
Note: This function is only relevant for private groups.
The rejectjoinGroupRequests() function returns a Promise that resolves when the site member's request to join a group is rejected.
Only site admins or group admins can reject site member requests to join the group, unless the group setting, membersCanApprove is set to true.
ID of the group requested to join.
Rejection data.