Adding Contact to a Group in the Contact App

A long shot but try the following: oot into Safe Mode according to Start up your Mac in safe mode - Apple Support and test to see if the problem persists. Reboot normally and test again.

NOTE: Safe Mode boot can take up to 3 - 5 minutes as it's doing the following; 

• Verifies your startup disk and attempts to repair directory issues, if needed

• Loads only required kernel extensions (prevents 3rd party kernel/extensions from loading)

• Prevents Startup Items and Login Items from opening automatically

• Disables user-installed fonts 

• Deletes font caches, kernel cache, and other system cache files

Is this a normal group, ie, not a smart group? Smart groups are defined by the set criteria. Normal groups work just as they always have.

Please run Etrecheck and post its full report here.

Use the “additional text” button and paste the report into the text box.

I see none of the usual suspects in the report. Looks clean to me. I'd reboot more frequently, line at least every other day to clear out temporary system and application swap and cache files.

You should try the Safe Mode boot next.