Clinical interventions have been effective at increasing social skills of youth with autism spectrum disorder (ASD). However, generalization of those skills to non-clinical environments is often low. To reduce this generalization gap, community-based programs have been designed to help youth develop social skills in naturalistic settings. This paper describes a community-based social-skills intervention for youths with ASD aged 7-12, which was designed to build on the findings of a previous study. In this program, youths with ASD co-operated with siblings and peers in structured and unstructured play over the course of four weeks. The researchers conducted extensive observations of the play by the youths and conducted interviews with the youths with ASD, their parents, and program staff. Both in our observations and in the perceptions of the parents, the youths with ASD increased their play and socialization. Using Vygotsky's (1978) socio-cultural theory as a guiding framework, we describe the components of this intervention that contributed to the changes observed in the youths' play and social interactions. We developed a model that includes components of instruction, interest, play- based learning, and the structure of program, and which provides an explanation of intervention effectiveness to be explored in future research.