If a user is invited to collaborate on a folder and they do not accept this collaboration invitation(from the email notification received or the Pending Items Manager in the user's Box account) then the collaborative folder will not appear in the user's Box account until the collaboration invitation has been accepted. 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, please submit a support ticket for assistance.
If a collaborative folder that you do not own is missing from your Box account, it is most likely that the owner of the collaborative folder has removed you as a collaborator on the collaborative folder. You will have to contact the folder owner of the collaborative folder to have them reinvite you to that folder again.
All users collaborating on a collaborative folder who have an Editor access level or above on the folder will be able to move, edit, add, and delete contents within this collaborative folder. If you notice that content is missing or in a different location within this collaborative folder structure, it is most likely that another collaborator on the folder moved/deleted the item from its original location. If an items was deleted within a collaborative folder by another collaborator on the folder, the deleted item would show up in the trashs' of the user who deleted the item and the folder owner of the collaborative folder.
If you invite someone to collaborate on a folder with you that does not have a Box account already associated with their email address, when they accept the collaboration invitation from the email notifying them on this invitation they will be directed to sign up for a Box account. Please note, it is recommend that users create Box accounts before being invited to collaborate on a folder.