Recently I was building in a group working on an area recently cleaned up by Operation: Clear Skies and after getting a few layers of the renovated area complete, we noticed that none of the blocks were properly being protected, even though we had group build mode enable. This is where my suggestion comes into play: I believe that group build mode should also enable personal build mode. The reasoning behind this is that the entire name "Group Build Mode" can be a bit misleading, making players believe that all the blocks they're placing down are protected, while in actuality only the blocks that are protected by default are being protected to the group. Also, I can see this being a player setting to toggle whether group build mode enables personal build mode upon joining a group or it being toggle on while in a group.
I'm a bit curious to see what this would entail. Unfortunately I do not know much about buildmode, but I do know that, as with any system, it does have it's share of glitches. (Which I have experienced, unfortunately... :\) If the bugs were ironed out of the current system, then I can see this being feasible. But, with the weird glitches the current system can suffer from, adding this level of complexity could cause even more issues. Love the idea (based off of my very limited knowledge of buildmode haha) but I hope that bugs get ironed out more before anything else gets stacked on top of it.
Currently, all group buildmode does is make it so any blocks placed in buildmode are able to be broken by any member of the group at the time it's placed.