The cost for you is marginal or non-existent, and you get an excellent referral system.
When you sell a membership, give the new member a bonus membership (one, three or even more months) he can gift to anyone he thinks might find it useful. You can test it as a surprise after they already joined, or as another incentive to join.
Continuity programs give you stable and predictable income. This might help convince people to join.
Offer parts of what you provide in your continuity program (webinar, article, one-day forum access) for non-members for a one-time access fee. You should price the single access fee to make your monthly fee seem very attractive. So if your monthly fee is $79, you might want to offer a single access for a one-time webinar (that you would get for free weekly as a member) at $49.