In this paper we present a web-based group formation tool that supports the instructor to automatically create both homogeneous and heterogeneous groups based on up to three criteria and the learner to negotiate the group- ing. Moreover, the instructor is allowed to manually group learners based on specific criteria. A discriminative fea- ture of this tool is the use of the Fuzzy C-Means algorithm for homogeneous grouping, which provides for each learner the probability of belonging to different groups. This infor- mation is also provided to the instructor to support him/her in manually exchanging learners or intervening in the ini- tial grouping. Moreover, the learners are informed for the groups formed and they are allowed to negotiate their group assignment. Preliminary evaluation results provide indica- tions for the efficiency of the proposed approach in forming homogeneous and heterogeneous groups in a real context.