Hi @rebecca1,
No, they will not have to pay to upgrade.
X7 and X8 users are able to collaborate with each other via the Library Sharing functionality first introduced in X7.2, released in September 2014.
Meaning, the following statements are all true:
As an X7 user, I can share my library with 14 other X7 or X8 users.
As an X7 user, I can have an unlimited number of libraries shared to me by X7 or X8 users.
As an X8 user, I can share my library with 100 other X7 or X8 users.
As an X8 user, I can have an unlimited number of libraries shared to me by X7 or X8 users.
Best of luck on your project!
Meredith M.