Startups as opposed to open source? I suppose that would cut down on the users, but how would you police it? To be honest I think that would just annoy people,

Startups as opposed to open source? I suppose that would cut down on the users, but how would you police it? To be honest I think that would just annoy people, as well as being a lot of work for you. If you really want to limit signups, I would just allow only X signups, or Y signups per day. That way you're being honest with your users: the message is "We're in trial mode, so we're only taking a few users now". People will understand that I think.