These guys did a great job of tracking UDFA signings last year, and it looks like they're on it again this year: newpatriotsdraft.com
According to them, so far SD has signed:
Jeremy Williams, WR, Tulane
Nic Richmond, OT, TCU
Brandon Lang, OLB, Troy
I won't be able to sit this thread, so add updates in the comments (or maybe a mod can take it over).
This site is unofficial and the data changes frequently, so make sure you have a grain of salt handy before you check the link.