I cannot add an existing contact to an existing group by dragging it to that group.

The contact should be created in Mac address book , under section All iCloud , select the contact ( use command and A keys ) , copy it ( use command and C keys ) , and paste ( use command and V keys ) to a group created .

It seems you trying to drag and drop contact from iCloud.com to a group that feature will not work .

Hi Stephen353,

You can still drag and drop contacts into a group, unless it is a Smart Group. As those groups are created via rules you set, you cannot drag and drop contacts into them.

See more info here: Create and change groups in Contacts on Mac - Apple Support

Add contacts to a group

1. In the Contacts app  on your Mac, select one or more contacts listed in the sidebar.

There’s no limit to the number of contacts you can add to a group. If you use more than one account in Contacts, make sure the contacts you select are in the same account.

2. Drag the selected contacts to a group in the sidebar.

If the contacts aren’t added to the group, check if it’s a Smart Group—you can’t add contacts to a Smart Group.

If you use an Exchange account in Contacts, those contacts can belong to only one folder at a time. Any contacts that aren’t in a folder you create are in the default Exchange Contacts folder.

Cheers!