Group by User Field with Multiples allowed

I have a column within my task list (SharePoint 2010) that is populated from my Global Address List. When a client calls us, I am hoping to use this information to find a list of tasks that we have worked on for that client. This field must allow multiple entries as there will be multiple points of contact per task.

I want to be able to somehow pick a name (pull-down...search...anything) and see a list of tasks that their name is associated with.

I am unable to do any kind of filtering/sorting due to the multiple entries being allowed.

Am I missing something obvious?

Thanks!