There are a few problems which startups seem to face in the process of rolling out invitations. In the case of Google Wave many people with an invite discovered they had non of their friends on the service to interact with. This is not beneficial for the user experience of your new service. Another problem is seasonality in user activity. Startups receiving a lot of publicity when a new invite batch is released.  Shortly after the buzz has passed user activity flattens with only some active users remaining.

Both problems can be solved by focusing on the social experience of users in the beta invitation process. Beta invites should be send out in batches of two, just like concert tickets in a prize giveaway. This can increase the probability of users to remain active by channeling their social behavior through your service. (I only became an active foursquare user after finding someone offline to share the experience with.)

Something to think about and which should be fairly easy to integrate in the invitation process on your website. What do you think?