Well, I always liked the way that RC5 did the WUs with teams.
Any WUs crunched by you were credited to you. If you had never been on a team and joined a team, the WUs also get credited to the team. If you leave that team and go join another team, any WUs crunched for the first team still are credited to that team. You also get credit for those WUs in your personal stats.
Any new WUs crunched are credited to the new team.
So the team gets credit for any work done while you are a member, you always get credit for work done whether or not you are on a team, and if you change teams, any new WUs are credited to that new team.
That way, one person with 200,000WUs can't impact a team's standings if they decide to leave like happens with SETI.