After some investigation this seems to be a pretty well known issue. However I had not come across this issue so I thought I would Blog it so I can easily find the solution again.
Although there are work arounds for this issue, you will be happy to know this will be fixed and is addressed in KB #919232
When the TFS Client is installed, Microsoft Project like Excell becomes connected to the Team Foundation Server via a new tool bar in Project. Using this a Manager can read or publish work items from the TFS Project. The problem, is when the users on TFS are defined in Active Directory the AD friendly name is Lloyd, David. In MS Project multiple resources can be assigned to a task using a separator. Which by default is a comma.
Here in lies the problem. When you take a project and try to publish tasks with multiple users to TFS you will get an error, telling you the user is not defined. When you read tasks from a TFS Project it will work but you will end up with multiple users. (Lloyd and David)
There may be more than one solution to this, however I found a simple solution on the MS Forums which is to simply change the separator Character in your Regional Settings so Project does not use a Comma as the separator.
If you're not sure where this is follow these directions:
From Control Panel open Regional and Language Options
On the Regional Options tab click the Customize Button
On the Numbers tab change the List Separator
There are other work arounds but they are not even worth mentioning.