Getting back to the issue of having archers who are not affiliated to ANY archery organisation...
One option would be to constitute an archery club in its own right, separate from the rugby club, consisting only of members who are affiliated to the
GNAS. Then form an agreement with the rugby club that all members of the archery club will also be members of the rugby club and will have access to the rugby club's facilities. This will allow you to tell the
GNAS that all members of the archery club are
GNAS members, while retaining the relationship with the rugby club.
The non-affiliated archers would still be members of the rugby club.
Then to avoid insurance problems, keep minutes of when the archery sessions include only
GNAS members (and so are covered by the
GNAS insurance) and when archery sessions include archers who are not in the
GNAS (and so are covered by the rugby club's insurance). These minutes should be made in advance of the sessions taking place. If you're not bothered about having
GNAS insurance at the club, you could just make one minute stating that all archery sessions will be covered by the rugby club's insurance and not by
GNAS insurance.