Purpose: Despite group-level improvements in active engagement and related outcomes, significant individual variability in response to early intervention exists. The purpose of this preliminary study was to examine the effects of a group-based Naturalistic Developmental Behavioral Intervention (NDBI) on active engagement among a heterogeneous sample of young autistic children in a clinical setting. Method: Sixty-three autistic children aged 24-60 months (M = 44.95, SD = 10.77) participated in an inclusive group-based NDBI over a period of 10 months. Speech-language pathologists used an abbreviated version of the measure of active engagement to rate children's active engagement at three treatment time points. Results: Linear mixed-effects regression analyses revealed that active engagement significantly increased from Time 1 to Time 2 (after 6 months of the group-based NDBI) and persisted through Time 3 (after 10 months of the group-based NDBI). Symmetrized percent change analyses revealed that 48% of the sample (n = 30) exhibited an increasing trajectory, 29% were stable, and 24% showed a decreasing trajectory. Age and parent-reported social pragmatic concerns at program entry, as well as the length of time participating in the group-based NDBI, were differentially associated with the identified subgroups, signaling baseline child characteristics that may be associated with NDBI response. Conclusion: These findings highlight the importance of careful monitoring of active engagement to guide clinical decision making regarding changing intervention strategies, targets, or the intensity of the NDBI if gains are not observed.