The contemporary workplace is characterized by all kinds of (synchronous and asynchronous) human interactions both within and across organizations like meetings, discussions, consultations, conversations etc. Although such interactions are an essential part of a modern organization's business processes, current research on business processes focuses mainly on the sequence of activities or tasks within a process (i.e. workflows), and not on human interactions. This research presents an application of an interaction-centric process modeling language named TALL for collaboration processes (CPs). For the illustration and evaluation of the language, a case study of a real-life CP is presented. The case study concerns the collaborative work of a Dutch municipality in relation to young persons who drop out from school. Shortcomings of existing workflow-based languages in the modeling of CPs are elicited. In the case study, the language is shown to address these shortcomings. The main strengths of TALL are (1) the visualization of human interactions, their roles and agents, and their relationships in a process-oriented tree diagram, and (2) the explicit recognition and specification of the local behaviors of the agents who are assigned to play the roles in a given interaction. Evaluation results are discussed. These results comprise practical insights, and contributions to theory and language development.