As young as 3 years old, children rely on a mutual intentionality framework to confer group membership--that is, agreement between a joiner ("I want to be in your group") and group ("We want you to be in our group"). Here, we tested whether children apply this cognitive framework in the context of identity-based groups, specifically gender and race. In Study 1 (preregistered), we asked a large sample of 3-8-year-olds (N = 448; 224 girls) whether a novel joiner character (girl, boy) could join a group (girls, boys) based on joiner-group intentions (non-mutual, mutual) and joiner-group gender congruence (incongruent [e.g., girl-to-boys], congruent [e.g., girl-to-girls]). Study 2 (preregistered; N = 433; 208 minoritized race) followed the same structure as Study 1 but instead varied the race of the joiner (Black, White) and group (Black, White). In both studies, participants as young as 3 years old relied on a mutual intentionality framework to confer group membership. This effect strengthened with age, replicating past work and newly showing that children rely on mutual intentions in the context of identity-based groups. An exploratory integrative data analysis (IDA) comparing across studies revealed that participants additionally relied on joiner-group gender congruence to confer group membership as young as 3 years old (Study 1) but did not rely on joiner-group racial congruence until 5 years old (Study 2). It appears, then, that young children's determination of group membership is influenced by interactive cognitive processes that incorporate others' mental processes (intentions) and their emerging understanding of the social world (identity-based group boundaries).