There are algorithms for determining how to do round-robin match play for N teams when you play fewer than N-1 games.  I don't know all the details of them but they are out there.  One possibility would be to pair up people with similar records (first round being random), then changing the pairings so you don't play someone else more than once.  They do stuff like that for chess tournaments all the time, there simply isn't time to play against everyone in the field.

SH