Add a user group.
API method: POST /v1/addUserGroup
Tip: To add users to a user group, use the linkEntities method, specifying the user ID and the group ID. It does not matter which entity is used as the user ID or the group ID. Either of the following Java examples will add a user to a user group:
void linkEntities ( long user_id, long group_id, String properties ) or void linkEntities ( long group_id, long user_id, String properties )