I use the MSR groundhogs as well. I use the stock line on the tarp and run it down through the loop on the MSR. I then loop it back up through the second tie point (the little clip) then tie it back to the 3 lines below the tie out points. This tensions it as well as triples the amount of line taking the tension.
I'll try to post some pictures or a quick video later this week when the weather improves