A person can act as a resource to a full-time team without being a member of the team. The team can contract for deliverables, consulting, review, or time. After you understand the nature of the work and the contribution needed, work with the full-time team and the person whose skills are needed to develop explicit agreements. Don't rely on happenstance and assumptions. Reach agreement on how the full-time team and part-timer will work together, how the part-timer will contribute, and how the full-time team will keep the part-timer in the loop. Don't expect he'll gel completely as a member of the team—because he won't.
Sometimes I see groups in which almost everyone is part time. Sometimes a group working together part time will gel, usually when they have a long, shared history. Don't expect groups made up entirely of part-timers to gel as a team without time and attention to creating a cohesive whole. And as long as the group is accomplishing its goal and managing the interdependencies between tasks, that's okay.
It may look simple on paper to say a team needs a specialized skill X amount of the time, but by analyzing the nature of the work and being explicit about part-time arrangements, you will help the team and the part-timer work together more productively.