As a student PPL I consider myself lucky to be in the fortunate position to be able the share the costs of flying with a qualified pilot. Well, OK..Its not me doing the flying (except for the odd S&L), but I get to experience navigation, RT, good airmanship required and a host of questions answered.
These shared costs are considerably cheaper than lessons but the observational experience gained in a more relaxed environment can only be beneficial to the learning process, and may, in some cases, mean fewer lessons in the long run.
It has also given me experience of other airfields and Pprune Fly-Ins ! I expect to pay my share as both pilot and passenger both benefit from this arrangement.
In my opinion if you invite friends who know you well and may know your financial position, they would also expect to share the costs ; especially if they have the choice of where you fly.