I think this is a fairly common situation. When trying to arrange something, some people don't like to commit so they say "maybe". This is a problem, because space may be limited or reservations may need to be made.
Here is an example. I would like to go to a concert. I asked a friend who said "he should be able to come". If he's not able or doesn't want to come, that's fine, I will ask someone else. I just want a straight answer. How can I get this?
It gets even more complicated when a group is involved. At college I invited a few friends (who didn't know each other) to join me at the pub. Two showed up. They had entirely opposite personalities that weren't really compatible and they didn't like each other. This is another reason I like to know in advance who is coming. Though I do feel a bit immature in this line of thinking?
I don't think this is the same as How to get somebody's confirmation when they say "I'll check my schedule"? because it focuses on people saying "I need to check my schedule". In my situation people say "maybe" or just keep asking for more details.
