This study investigates what factors affect the group size of voluntary contributors to a collaborative task in wiki-based online communities. This study proposes work popularity and online resource availability (scholarly reference and news) as the determinants for group size. The results of the empirical test using the data on 876 English Wikipedia articles show that online resource availability significantly determines the group size, and influences the group performance via the group size. However, work popularity does not affect the group size. The interaction for discussion among group members shows the significant effect as a moderator for the relationship between the group size and group performance.