1. Use the Tours → Groups item from the main menu.
  2. Use filters to find desired groups.
  3. See column named Total number of seats.

NOTE: This number shows the sum of the total number of seats that all of the subgroups of this group together have.

HINT: It is possible to manage visibility of columns if some are not included. Go to Edit columns button the right side and choose this column to be in “Included columns”.