The SocialWorlds group usually consists of 4-5 PhD students and a small number of undergraduate and masters students.
I admit roughly one PhD student per year. Sometimes students are co-admitted or co-advised, so the number of admits can vary.
For PhDs, my group is pretty evenly divided between technical and social people. I especially like students with a foot in both camps - people who want use social analyses to understand how to design, people who are technical who want to understand the social side of the world, or people who are social who want to use new data techniques. There are no set criteria, but you're much better off towards admissions if you've contacted me and let me know of your interests and goals. Make sure to look over these pages carefully; the match should be pretty strong. A PhD student is very costly - in time and money - and I select students for my research group carefully.
I admit candidates both through the School of Information and through Computer Science and Engineering. The criteria are different; you should check the respective web pages for basic requirements. The College of Engineering is quite stringent about their minimal requirements. The School of Information is less stringent on the math GRE, but likes higher verbal GREs than does Engineering.
For masters students, I generally cannot give you a graduate assistantship. There are exceptions but they're rare and they come after you've volunteered for a semester. Master's students are more caught up in coursework, and to work with the SocialWorlds Research group, you need to be committed to finishing projects and doing research. That having been said, some great masters students have done projects with our group and then been admitted to PhD programs, and I've had some excellent students carry forward their class projects.
There are usually 1-3 undergraduates hanging out on various research projects. I enjoy working with undergraduates. I do look for some serious work and effort associated with your research project, but in turn, if you want a recommendation letter later, it will reflect that effort. We have the easiest time working with technically- or design-oriented students, but occasionally we've had undergraduates working under a PhD student on a social project. You will be expected to join our group's research meetings.
Both undergraduates and master's students have been co-authors on papers.
As with PhD candidates, look at the various projects on our group web pages (www.socialworldsresearch.org/projects) and see whether there is something that could fit.