Grouping reuses the room you build in the Seating Chart — the tables and seats become your groups. Lay out a room there, then come back to form small groups.
This is your Seating Chart room, with empty seats. Pick a grouping type and click Generate groups. Groups fill the tables: at group size 2 a 4-seat table becomes two pairs, one on each side; at a size that fills the table, the whole table is one group. To change the room, open the Seating Chart — your saved seating assignments are never touched here.