Find a small project (1-2 wks of work) you can work on together and make it clear that it is a trial process to see how you will all work together; something other than your startup (for possible legal reasons in the future). If any of them are unwilling to do this, that should be your first signal that that person isn't right for you.

Also since they came up to you after the presentation of the idea I would definitely make sure they know that whatever you are doing isn't the "one true idea" as it stands now, since it is likely to change significantly as you continue to work on it.

The "just go for it" is advice lite and I wouldn't take it, at least not for choosing your team, which is likely far more important than your idea.