There are two things that I think go a long way in making a scuba club successful.
1. You need a strong shop presence. I know not all but most scuba clubs are associated with a local shop. I can speak from experience when I say that the more successful clubs are going to be clubs where the shop owner, staff, instructors, and DM's buy into the importance of the club, attend the club meetings, and promote the club to new students. In most cases, the shop is the life-blood for the club.
2. Club members need to be very outgoing to new members. Again, speaking from experience, it makes all the difference in the world when a new guest or visitor comes to a club meeting and several members of the club introduces themselves to the guest, makes them feel welcome, and invites the guest to sit with them or something like that. People will come back if they feel they are wanted back.