I see nothing wrong with inviting study groups, research groups, or even small groups of 2-3 students over for dinner, on outings, etc. In fact, I think it's perfectly appropriate, and a great idea.
The issue in my mind comes when you are having INDIVIDUAL meetings in social contexts, with either employees or students. The first question to ask yourself is, "could this be construed as a date". The second question is, "if I wouldn't have this meeting with someone in my romantic attraction group, because it wouldn't be appropriate, then am I unfairly disadvantaging my students who are in that group"
The issue in my mind comes when you are having INDIVIDUAL meetings in social contexts, with either employees or students. The first question to ask yourself is, "could this be construed as a date". The second question is, "if I wouldn't have this meeting with someone in my romantic attraction group, because it wouldn't be appropriate, then am I unfairly disadvantaging my students who are in that group"