One woman I know who makes a lot of sheva brachos tells the guests (and the chosson and kallah) that her husband has to leave for his shiur at 8 p.m. (of course, that only works if your husband really does have a shiur at 8 p.m. as hers does). If they don’t bentch by the time he has to leave, they won’t have a minyan.
This usually works.