The idea is indeed that people arrive Friday and leave Sunday/Monday. Although of course you can come earlier. As for me, I'll probably be in Groningen from Thursday night until Monday night. People staying in my brother's house are kindly requested to leave on Monday, because my brother has to use his house again by then :-)
I've just had word from the youth hostel:
they have dorms with 16, 18 or 20 beds which we can book (but if we want the whole dorm we pay for all the beds), and smaller rooms for 2, 3 and 4 people. The youth hostel has lockers for your instruments, is close to the city centre, open all night and inexpensive. I think I should be brave now and make a decision: instead of camping, let's try the hostel. Could I have names ASAP please? I'll then make a reservation within the next two weeks. I could also recommend hotels and b&b's, but let's get those reservations going!