Can't add to groups in the Contact app on Macbook Air
Using Monterey version 12.7.6. and MacBook Air. Contact app will not allow me to add new contacts to groups.
MacBook Air 13″, macOS 12.7
The method is to drag a contact from the list over to the group in the sidebar.
Is that that the process you are trying to use?
Create and change lists in Contacts on Mac - Apple Tugi (EE)