Group-building for project, how to handle non-response? I recently switched to a new university and our professor asked us to make groups for a big project next semester. Since I was new I sent an e-mail to everybody available if they are free.

I got a response and answered with the topics I wished to pursue. I haven't heard from him since 4 days.

In the meantime somebody else contacted me with matching interests in topics.

What is a professional way to get an answer from the first student? Should I just set a deadline?

How can I handle the second student the best way?
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.