  Multiplayer online battle arena games provide an excellent opportunity to
study team performance. When designing a team, players must negotiate a
\textit{proficiency-congruency dilemma} between selecting roles that best match
their experience and roles that best complement the existing roles on the team.
We adopt a mixed-methods approach to explore how users negotiate this dilemma.
Using data from \textit{League of Legends}, we define a similarity space to
operationalize team design constructs about role proficiency, generality, and
congruency. We collect publicly available data from 3.36 million users to test
the influence of these constructs on team performance. We also conduct focus
groups with novice and elite players to understand how players' team design
practices vary with expertise. We find that player proficiency increases team
performance more than team congruency. These findings have implications for
players, designers, and theorists about how to recommend team designs that
jointly prioritize individuals' expertise and teams' compatibility.
