Group-building for project, how to handle non-response?
First stop would be to get a phone number of the student - you might be able to get it from the university if you can't from a friend or classmate. Explain the situation to the student and I'm sure they'll understand - it's a common enough thing to happen. 

If you can't get a phone number or feel it would be inappropriate (I honestly don't think it is in a time critical scenario), then send another email with a deadline. If you wanted you could put a read receipt on it so you'll know if they've opened it - some people dislike this though as it feels a bit intrusive. 

In terms of the second student, explain the situation fully and say you've got a deadline - ask if they'd mind waiting till them, but you can't expect them to wait if they get a better/more suitable offer.