Charging per user B2C app

Hey hackers!

I am currently prototyping an app that allows individual users to create a free one-off project for themselves, but if they want to collaborate with others and keep track of all advantages that come whilst collaborating, they can invite others to join- but this will cost them money.

I am not sure I've ever seen an example wherein the B2C space users need to pay for inviting others. I can't really figure out how this would work with different tiers depending on the number of other users in that project, but I would love to know examples of other (saas) apps that have this same business model in the B2C space.

Your input is really appreciated!