A collaborator is an individual who has been invited into a folder. Collaboration invites are a great way to share content for long term projects. You can invite people to a folder and monitor updates over time. When a user is added to a folder as a collaborator, the folder will appear in their All Files page. If you send a collaboration invitation to a person without a Box account, that person will be prompted to sign up with Box before being able to access the content.
Follow the instructions on the link below:
https://community.box.com/t5/How-To-Guides-for-Sharing/Collaboration/ta-p/19723#invitingcollaborators