Hopefully the Story is divided into multiple demonstrable Outputs and then to specific Tasks/Features. If this is the case, then based on the worklog done & completion achieved on the specific Task/Features by the assigned team-member, his/her velocity could be calculated from these. Hence grouping all the team members outputs completion (tasks/features), Team’s velocity could be calculated i.e., numbers of work/task/features completed in a sprint/milestone/release.

Thanks
Sandeep