Once you create the test, you can open it up for applications. You can also search based on the demographic you want (age range, geographic, etc.) and invite users yourself.

If people apply, you can pick from those. I invited about 60 testers and had ~180 others apply. I was able to pick from that 180 set - and the reward goes only to them.

Also, before you start the test, you buy 'Beta' credits. You can choose to use these however you want, for example if you buy 50 beta credits, you can give a $5 reward to 10 testers, or a $10 reward to 5 testers. You can buy more credits as you go if you want to offer rewards to more testers.