What does "on-site" mean in the following context?
That evening, relax. Let everyone unwind, but on-site. Have an informal full-group dinner or some other social event that lets everyone talk informally...
It means staying in the same location. In this case, if people have been working that day in a conference centre [for example], then the full-group dinner or other social event should also take place in the conference centre, rather than going out to a restaurant.
I'm not a teacher of English, but I have spoken it for (almost) all of my life....