Well there really is no official way to do this. What everybody normally does is go to the engineering buildings and look for a free class room and just settle in. It is normally never an issue if we keep the place tidy when we leave. The custodians come and check up and clean the rooms but they are very polite and never cause any issues. In face they are always curious about what we are doing.
On the odd chance we do get told to leave, its always because there is a class coming in or somebody has booked the room. But in those scenarios we just pick up and move to another room where we continue what we were doing.
Ofcourse, there is a chance (though this has never happened to me), that all the rooms are busy. In this case I guess our meeting wouldn't go through. However the odds of that happening are super, super slim.