How do I add contact to a group in Contacts

open the contacts app

in the menu on top of the screen select VIEW

select SHOW LISTS

your contacts window should now show the groups on the left side.

drag the contact to the group

that should do it

I guess you tried dragging it into the group and it didn’t work?

If so, try creating another group and see if that works. If so, drag the contacts from the old group into the new.