We investigated socially inclusive participation in mainstream community groups and religious services by U.S. adults with intellectual and developmental disabilities using weighted secondary analyses of 2018–2019 National Core Indicators data. Overall, 34.4% participated in community groups and 42.4% in religious services. Some 45.0% had an unmet desire for community-group participation, whereas most (75.0%) attended a religious service as often as preferred. The type of companion varied by living arrangements and age group. Attending community groups and religious services were each strongly associated with better friendship outcomes but were not related to loneliness. The large unmet demand for community-group participation reveals a major gap. The friendship outcomes underline the benefits of socially inclusive community participation.