Cake and/or Bacon rolls are always a winner.
Honestly though the best thing you can do is just create a friendly atmosphere, have a committee that is approachable and engage the members, that way they feel more a part of things and help willingly.
I've noticed in the past that when a club has a core who do everything, the perception outside of that core is often that they are a kind of clique. Usually this is perpetuated by the core members shooting on the same bosses alongside each other and/or acting a little like supervisors doing all the range duties.
I'm not accusing your club of being like that, but if it is (however well intentioned) then try getting the core to mingle more with the non-core on the line (as equals, not just offering advice), set up a rota for field captain duties whereby everyone has to take it in turns step back for a few ends with the whistle (gets everyone involved).
One of the most useful ideas we had at a club I go to (a rifle club rather than archery but similar situation) was to hold a pairs tournament. Shooters were semi-randomly (top half in one hat, bottom half in another, one from each hat make a pair) drawn together and then given a fairly low target score to aim at with lost points for overshooting. It forced partners to talk to each other who would normally rarely mix and we found it made the club much more sociable.