Hi Kelz1004 and welcome,
I talked to Clayton Cobb about this and we came up with one idea to get all users in SharePoint. Basically, you're going to want to just add a data connection to retrieve data from an XML Data Connection and type in http://www,domain.com/_vti_bin/owssrv.dll?Cmd=Display&List=<X> where <X> is the User Information list in SharePoint. You'll need to get the actual string from your ShP site. It will come encoded with GUIDs etc. After you add that, it will get all of the users and automatically populate on load of the form showing you all users. Keep in mind that if the number is large, it could be a performance nightmare.
You may also be interested in Qdabra's Active Directory Web Service which lets you automatically populate users in a dropdown based on group, manager, or substring of their name. It supports filtering and is fast. It also comes with an hour of free support. Go to Qdabra.com and Products and click on Active Directory Web Service.