We have a couple of places in villages near us that hire out barns, I definitely know wedding receptions have taken place in those. Something like that would be perfect for what you want.
I think if someone is coming along last minute and sleeping on a put up bed or sofa, I wouldn't expect them to share in the accommodation price significantly.
I think they will be completely unaware of this and actually really embarrassed when they find out. Sounds like they have probably paid for this app and not realised that all of the guests will have to commit to a 'free trial' (where you enter your bank details ready for payment to kick in at...