Online Appointment Sign-Up Site:

This semester, I'd like to automate my students' signups for various tasks (office hours, lunches with me, and the like). It would be great if they could go to some site, see the available spots, add their names for the spot that works for them, and move their appointments if necessary.

I imagine there must be some such services around, either free or costing a modest amount. I'd prefer ones that require a minimum of registration hassle, both for me and for the students -- for instance, ones that just give me one password that I can hand out to all the students, but that will let the students sign up for the available time slots under their own individual names. (Not the best security, I realize, but I don't need top security here.) If you folks can give me some tips on this, I'd be much obliged. Thanks!