Some people have a tight schedule and others don't. Some people are not happy about staying after class and having a "chat" because their wife is already upset about them spending so much time away from home — with work and all. Students may have to study, and others may not be "the social type."
In any case, I don't like people stealing my time and I try not to steal others' time. So, if I want people to get together a little more, we bow out of class a little early and sit in a circle and have a short chat until the time that class would normally end.
That way, nobody feels like they're being forced to stay after class, because we end our chat session at the same time that class would normally end.
If the dojo is large, chat sessions may be divided into mudansha and yudansha.
I especially like to have a short chat sessions at the beginning of the semester (I teach at a college), after mudans take a test (usually at the end of the semester), when there's club matters to be discussed, or when I want to discuss a particular topic (such as dojo etiquette).
While I think such sessions can help the atmosphere of the dojo, whether it makes people into "one big happy family" is questionable.
It seems that every dojo has its own spirit, and that spirit can change over time. A big city dojo with lots of students will clearly have a different feel than a small-town dojo with 12 students. A lot also depends on the spirit of the students and the teacher.
I spend a lot of time preparing myself ahead of a class. The night before, I may review what I have to teach. I may look at books to review waza in order to gain some small insight.
I take a 50 minute train-ride to the town where I teach. During that time I again review what I will teach, then read a short zen story and meditate.
I find that when I am well prepared for a class (in mind, body and spirit) it has a very strong positive influence on the students. This can be the first step in developing the kind of dojo atmosphere that you are seeking.
Although we can control ourselves, trying to control others just ends up being an exercise in futility. Therefore, it's important to have a clear understanding of the limits of what we can do. At least, this has been my experience.
Conditions are always changing, so I try to develop a sense of comfort within myself through my daily physical, mental and spiritual routines. That way, no matter what happens (whether things go my way or not), I can handle it.
