Yes, it is possible to merge existing group reservations and create one group reservation.
Note: Merge group reservations are only possible when the Booking status is a reservation. once the guest is checked-in we can not merge group reservations.
Please refer to the steps shown in the video and screenshots.
Path: Reservations >> Switch to group >> Reservations >> Merge group >> Select the groups >> Merge group >> select the new group owner >> Save