There are now new rules and a new application for quidditch.
New rules are here.
New application is here.
The following houses have positions open currently: (assuming everyone on the current roster is going to play next term)
Gryffindor: 1 Alternate
Hufflepuff: 3 Alternates
Ravenclaw: 2 Alternates
Slytherin: 3 Alternates
Just because it's an alternate doesn't mean you won't get to play or you won't get to play the position you want. It really is a first come first play type of system we have. So being an alternate isn't a bad thing!