[JonathanWCurd]

With your app it kind of sounds like you have a chicken / egg problem so you may want to just open it up to the most users possible so tat it is the "best it can be".

In the past we have tried both and I've found being more restrictive allows you to iron out things (most of the time unforeseen) before a full scale launch but you lack the large user base that you would get by just opening it up (Signup to be notified lists in my opinion and experience leave a lot of users out cold who don't come back).

In addition, larger betas can get you more traction quicker and also provide insight into new markets and uses for your app you never thought of while smaller betas on the other hand create a good core group of early adopters (because you can give them more attention) that if taken care of will help to champion your cause and spread word about your product.