It may seem obvious, but it's really a question with profound implications. Some may join for the benefits. For some it may be a belief in the mission. It may be the organization’s reputation, or the recommendations of others. Some may join just to meet others like themselves.
In what venue and under what conditions did they hear about the organization and decide to join? What influenced their decision and how long did they consider membership before signing up? And importantly, what are their expectations now that they’ve joined?
Information like this is valuable for future recruitment efforts; telling you what works and why. By identifying expectations, you can improve your chances of retaining new members.
The best way to get this type of information is to ask new members soon after they first sign-up, while the experience is fresh in their minds.