Here's the news nobody wants to hear: The problem is not with the part-time people, rather it's with the part-time assignment.

Priority
When a person is assigned to more than one team, she'll gage priority by the proportion of her time that's allocated to each project. One tester I know was assigned 90 percent to developing manual test cases and 10 percent of her time to creating a "critically important" automated test harness. No matter what words her manager said about the importance the test harness, the fact that she was directed to spend only 10 percent of her time on it communicated a different message. Furthermore, when she attended meetings or working session with the automation team, she spent most of the time catching up on what had transpired since her last sliver of time with the team. The arrangement wasn't satisfying for her, and frustrated the automation team. Soon, she stopped working on test automation at all.

Ambiguity
Many people will opt to spend their time on work that has clear-cut outcomes over work that's ambiguous. The more clarity around the part-timer's contribution, the more productive the situation will be for everyone.

Team Cohesiveness
If the full-time team is cohesive, the part-timer may not be able to fit in. Gelled teams develop their own subcultures. They may have unspoken rules of engagement, communication patterns, and established relationships that make it difficult for someone who isn't full time to fit. Over time a part-timer may acculturate, though the process will be slower than it would be for a full-time team member.

Missing Context
When someone works on a team part time, he is by definition missing part of the context of the team. While the part-timer is off doing something else, the full-time team makes decisions, solves problems, and exchanges information. While the full-time team may document and transmit major events, there are numerous small decisions and communication events that may not seem important enough to pass along but still affect the way the work is done. The adage "You never step in the same river twice" fits this situation. Each time the part-timer re-enters the team, the team has inevitably moved on from the last time the part-timer participated.

So, what can managers and team members do to improve the situation? First look at the work and how the part-timer needs to contribute. Here are some of the questions you may ask about the work:

Is his contribution time-limited, meaning that his skills are needed for several days, but not for the duration of the project? Maybe the person only needs to contribute at some critical point. Perhaps he is only needed for one or two iterations, when the team is working on a particular feature. Consider contracting for the person to focus full attention for those periods of time.

Can the contribution be batched so the part-timer interacts with the team for a day or two a week? Blocking out a day or two a week to work with the team is more effective than having the part-timer spend an hour here and an hour there with the team. This may require more clarity and coordination from the full-time team, but it will make better use of everyone's time in the long run.

Is the part-timer needed for hands-on work, or can he act as a reviewer, consultant, or coach to members of the full-time team? Sometimes the full-time team needs to use a certain skill a little bit each day or week, but needs the skill over all long period of time. In this situation, it could make sense to develop the skill on the team and have the part-timer act as an expert coach or reviewer until the full-time team members are self-sufficient. Teams who have all the skills needed for the work are often most productive.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.