When creating a group, a maximum number of members can be specified, or set to unlimited. Once a group has reached capacity, no more users can join the group until either one of the following is done:
- Someone with the Manage Groups permission increases the maximum allowed members to a higher number.
- Someone with the Manage Groups permission sets the maximum allowed members to "Unlimited".
- An existing member of the group leaves, thereby opening up a spot in the group.