In introductory CS theory courses, instructors sometimes give students the option to work in groups when completing assignments so they can discuss problems and gain teamwork skills. It is important, then, to ensure that diverse student populations, including those with different academic abilities, can all benefit from this optional group work. In this work, we investigate how lower- and higher-ability students, as measured by prerequisite grades, perform in self-selected, 1-3 person groups of different abilities. We found that, in a first-year course, lower-ability students can benefit from being in heterogeneous groups in terms of ability.