Views Dropdown List of valid items I'm looking to add an exposed filter that only contains existing user inputed value.  

For example.  I've create a content type called 'Items'.  Inside that content type there is a field called 'Team name'.  The user can freely enter the Team name and add multiple items associated with that Team.

When the user is viewing items, I'd like for them to be able to filter based on that Team Name, but rather than force them to type in the name, I'd like for the exposed filter to contain a list of Team Names that have been added into the nodes.

How to make an exposed filter that only contains existing database values?
It really sounds like the team name is a tag based on a Taxonomy Term. You could make a taxonomy called Teams and add a term_reference to your node type. Views can then make an exposed filter based on the term reference.

Using the autocomplete widget for the field will auto create non-existing team names. For more UI flexibility see Autocoplete Deluxe.