In Roomdoo, a group is a reservation with more than 4 rooms. The flow is the same as a normal reservation, but after clicking New reservation in the top-right of the calendar you pick “Group” instead. Then fill in:
The sales channel the reservation came from.
The rate that applies.
Check-in and check-out dates.
The rooms in the group.
From here, continue as you would for a normal reservation. See the guide on how to create a reservation in Roomdoo for the full flow.