First you'll want to navigate to the Membership of one of the Group members

Then you'll want to scroll down until you see the 'Group info' button, you'll want to click on that

Then you'll click on 'View Group'

Then you can the button that says 'Add existing member'

There you'll see two options to search for the existing Group member - the first option being the Members email address. You can search for the Members email to pull up their account

Or you can search via the Members Internal ID (not the ID that you can assign but an internal one that gets automatically assigned to each member)

Once you've searched for the member, you can then click the 'Add Membership to Group' button under the Membership you want to add to the group! You'll click 'Ok' on the pop-up and the viola the member is added to that Group Membership!