Hi @MarkSavantMedia, welcome to the Dropbox Community.
Since shared folders take up space in all members' accounts, it seems like the other user doesn't have enough storage in their Dropbox account to add the shared folder.
They can find more info about how to tackle this issue, here.
In the meantime, if you want to at least be able to view the contents of the shared folder in question, you can send them a shared link to it, instead.
Let me know how it goes or if you have any other questions, thanks!