Extended Reality (XR)'s focus is changing from single-user experience towards societal and industrial applications in which collaboration is key. However, most existing XR user experience research is mainly centered on the user's Individual Immersion and does not take characteristics of the group, such as Group Cohesiveness and its impact on Team performance into account. This is required, though, to allow for (near) real-time application-specific network and content optimizations for multi-user VR. I hypothesize that Group Cohesiveness in Collaborative VR (CVR) can be devised from three key concepts, namely Shared Cognition (SC), i.e. group-to-environment, Shared Awareness (SA), i.e. group-to-self, and Shared Task Engagement (STE), i.e. group-to-task. Therefore, this project proposes a hollistic, human-centric approach for the assessment and modelling of Group Cohesiveness in CVR and its relationship to Team Performance and Individual Immersion. To this end, three dimensions are put forward: (1) the realization of a task-driven evaluation testbed for gathering detailed evaluation results to fuel the thorough empirical user studies for substantial validation. (2) The creation of a subjective, objective, and physiological assessment methodology for SC, SA, STE, Group Cohesiveness and Team performance and (3) (near) real-time modelling of SC, SA and STE and their context-dependent contribution to Group Cohesiveness and Team Performance.