If I'd just gotten in and she was about to have someone over, I may want to be invited to join in
As long as no one's being left out I see no problem with it. Maybe some nights both feel like going out separately, but other times they may feel like sharing/introducing their partners to each other and relating on whatever level they care to at the time. If one person is much less sexual than the other, then it should be fine for the more vibrant one to supplement his/her sex life with others, while being sure to stay healthy in the process. With the whole open relationship/polyamory thing, each group sets its own rules as to what is okay.