It is very generous of those who are posting their tickets and committing to sharing the winnings. There can be a lot of confusion, and it gets more intense if big money gets involved.

When you say you are sharing it, do you mean you are sharing it with a limited group, or everyone who has seen your post and asks for a share later if you won? If you just post your ticket with no explanation, have you set any conditions on participation?

It is questionable whether those who did not contribute will have a legally enforceable claim on any prize, but you never know what a court will say.