create(), delete(), describeSObjects(), getDeleted(), getUpdated(), query(), retrieve()
A record exists for every User or Group who is a direct member of a public group whose Type field is set to Regular. User records that are indirect members of Regular public groups are not listed as group members. A User can be an indirect member of a group if he or she is in a UserRole above the direct group member in the hierarchy, or if he or she is a member of a group that is included as a subgroup in that group.
If you attempt to create a record that matches an existing record, system simply returns the existing record.