BuddyPress.org

> This requires that the site admin be able to add a validated member without the user log in.

So they can make the payment without signing up as a WordPress user on your site?
If they don’t have a login name, then they don’t have an ID and cannot be added to a group – by any method.

If they do have a WP login name, then you can add them in wp-admin.
Go to Groups, mouse-over a Group Name and click Edit.
On the Edit screen you will see this section: Add New Members.