If you wish to add reservations to an existing group, you have 2 ways to do this.
Merge 2 existing reservations into 1 group reservation
Open one of the reservations
Navigate to the "group" tab
In the "Merge Reservation" field type the group name of the reservation you are trying to merge. Once selected, click the "merge" button.
Add new reservations to an existing booking.
Open the original reservation to which you would like to add more bookings
On the right top of the screen, select the "Reservation" button (see image below)
Once you select this button the "New Reservation Screen" will open, which will allow you to create a new booking, which will automatically merge into this group.
You can find more information on how to merge a group here:
Reservations created via a channel manager can't be merged. Once the group merged, it can't be un-merged.