---------------------
 user_id  | group_id
---------------------
 1        | 6
 0        | 6
 0        | 6
 2        | 8
 3        | 8
 0        | 7
 4        | 9

---------------------------------------
 group_id | count_users | count_guests
---------------------------------------
 6        | 1            | 2
 8        | 2            | 0
 7        | 0            | 1
 9        | 1            | 0