Additionally, if a user does not confirm their email address during the sign up process, they will not see the collaborative folder they were invited to until they confirm their email address. If a user is having trouble confirming their email address, kindly submit a support ticket for assistance.

If you have other questions regarding collaborations, please take a look at this article: https://support.box.com/hc/en-us/articles/360044196073-Collaboration-Frequently-Asked-Questions 

Regards,