There are two ways in.
One option is to play in the live summer Game. The only requirement is that you are an intern employed by Microsoft. That's not a trivial requirement to meet, but you can read about the opportunity and, if it interests you, mail your school's recruiter.
The other option is to be a tester for us throughout the year. We do a rehearsal in the spring which is open to select people. Contact the leads for more information.