Individuals on the autistic spectrum and their families look for peer support in specialized online forums. These venues also attract advocates and people interested in autism, providing valuable first-hand experience. Previous research focused on quantifying how autistic individuals interact in online communities, and if they benefit from computer-mediated communication. However, there is limited quantitative understanding of the different roles that diagnosed individuals, family members, and neurotypical users play in these communities. This paper analyses Wrong Planet, a large online autism forum where users may openly state their condition in their profile. The sentiment, discourse, and network characteristics of content users contribute (and respond to) differs by user condition. Also, interaction patterns between users with different conditions shed light on the dynamics of the forum community. Content exchanges between family members and neurotypical users are emotionally charged and supportive; however, this support is less present in exchanges with diagnosed members. This paper gives insights on what factors facilitate participation of diagnosed users.