Although autistic adults may establish better dyadic rapport with autistic relative to non-autistic partners, it is unclear whether this extends to group settings. The current study examined whether rapport differs between autistic groups, non-autistic groups, and mixed groups of autistic and non-autistic adults, and whether differing diagnostically from the rest of the group results in lower rapport. One hundred and forty-three adults were assigned to one of four group types of four participants each: all-autistic, all-non-autistic, non-autistic majority (three non-autistic, one autistic), and autistic majority (three autistic, one non-autistic). Groups participated in a 5-minute building activity and afterwards completed a 5-item rapport measure assessing their experience. The all-autistic groups did not differ on overall rapport from the all-non-autistic groups and had significantly higher rapport on two items (enjoying the interaction and friendliness of the group) than both types of mixed groups. At the individual level, autistic participants expressed more ease and enjoyment when interacting with other autistic adults relative to non-autistic adults, and their rapport declined as more non-autistic participants were included in the group. In contrast, rapport for non-autistic participants remained relatively consistent regardless of group composition. We discuss potential reasons why autistic participants were more affected by group composition than non-autistic participants.