According to NFL Draft Bible, which is the most reliable source I can find right now, the Lions have signed these four undrafted rookie free agents:

FS Bobbie Williams, Bethune-Cookman

OLB Tyronne Pruitt, Boston College

QB Mark Nicolet, Hillsdale

RB Allen Ervin, Lambuth

I have been able to confirm that everyone listed above has in fact signed with Detroit other than Boston College's Tyronne Pruitt.  I'm still working on that, but the other three have reportedly agreed to three-year unguaranteed deals.

UPDATE: According to Fantasy Football Live Wire, the Lions have also signed these three undrafted rookie free agents:

Lions LS Jonathan Weeks (Baylor)
Lions OL Matt Butler (East Carolina)*
Lions S Justin Sanders (Illinois) tryoutwork