The speaker groups I created in the nest app were not manageable, post migration to the Google home home app. All of the solutions I've seen for this involved deleting/resetting speakers until the old groups disappeared (unable to delete them frome home app, and no longer visible in nest app).
Firstly check that any, all speakers that were in previous groups are powered on, then check again to see if management options have appeared, assuming you powered up a device that wasn't previously.
Any devices that you no longer have/cannot power on delete from your home, you can always add them back later.
Finally create a new group and add any speakers once this new group is saved the options appear for that new and the other old groups.
