Default posting rights of users from various sites?

In a multisite setup there are like 50 user generated sites.

UserXYZ signs-up on Site 10 to download a membership only freebie.

Site 25 also provides a freebie for it's members. UserXYZ wants to download that as well.

Now what he will have to do? Sign-up again or can we setup our multi-site in a way that a user signed up from any site is signed-up for whole network?