Teams Have Velocity; Team Members Don't
Tracking velocity at the individual level is a common agile implementation error. It often occurs when teams or organizations are misusing velocity as a productivity metric or management target. Please don't do that.
The point of velocity is to provide an aid to capacity planning. To be effective in estimating team capacity, all work done by the team needs to be included in the metric. Unless you've structured your user stories to treat these part-time resources as externalities (something not in evidence in your original post), their work must be counted as work performed by the Scrum Team.
If you don't include part-time members in your velocity metric, you are skewing the results. In particular, not including their work in your team metric will:
- reduce the team's ability to track the impact of part-time team members on overall team capacity;
- create "hidden work"; and
- cover up a key indicator of resource contention or organizational dysfunction.
CodeGnome's Law of Transparency says "No invisible work, ever!" That means that any work performed by any member of the Scrum Team must be accurately reflected in the trailing metric of team velocity, and properly accounted for when estimating current team capacity during Sprint Planning.