The best way to accomplish this is to
create two templates.
The first template would include all your unassigned shifts for M-F
and then the shifts for the weekend already assigned with the A group
(maybe name that template Week A).
Then make another template with the weekly shifts unassigned and the
B group set to the weekend rotation (and save it as Week B).
In future weeks then Import the saved template Week A and keep
the assignments, run AutoFill to fill the open, non-weekend
shifts. The next week use the Week B, then the Week A etc. to keep your
rotation.
Also note you can click the link at the bottom of every schedule week
"Schedule History" if you don't remember which template you imported
last. This will show the named template that was imported into that
week.
