This multilevel investigation examined the effect of group interaction and its influence on individual-level membership variables and group assimilation. The study is based on a model of group socialization developed by Moreland and Levine (1982) and was modified in this study to investigate the development and maintenance of highly interdependent workgroups in a high-reliability organization: a municipal fire department. Using hierarchical linear modeling, we examined individual- and crew-level influence on four assimilation outcomes: involvement, trustworthiness, commitment, and acceptance. At the individual level, acculturation predicted all the four assimilation outcomes. Involvement also was a predictor of the latter sequences of assimilation: commitment and acceptance. The study also found that one crew-level variable—crew performance—affected and modified the influence of tenure, proactivity, involvement, acculturation, and trust on members’ commitment. Implications are offered for the influence of group interaction on member assimilation and support for continuing group-level research on assimilation. This study also underscores the utility of multilevel analysis in examining communication at the interpersonal and group levels.