I don't really see a problem there. Just scan the last one. A griefer in Nukara can be someone who just stands by and lets others complete the "closing rifts" mission. It counts anyone closing them.
EDIT: When I'm doing Nukara, I'm just running around, shooting everything, scanning everything. It seems to be the best method to use. Then there is the less demanding possibility to stay close to one rift and clear anything that gets close while you close the rift every time it respawns.