That's an interesting question - how do the users interact? Can you have two of the same type of people interacting (a chat/message/game site) or are you offering different value to different users (a marketplace/scheduling/discovery site)?
If you're looking for one general type of user, go find a meetup group, class, online community, or other spot where they'll congregate, and just talk to them. Bring a laptop (or send a link) for them to test out, and maybe provide some sort of incentive or game to get them interested in giving it a shot.
If you're looking for two (or more) classes of users, do the same general thing for each separately, but either 1) try and line things up so that each group is interacting with the system at the same time (perhaps by only opening up the beta for a couple of hours) or 2) pretend to be a member of the other group during testing so that you can get feedback on at least half of your product.