Firstly, there may not be any bills for them to see in the Intray, but this is most likely due to the User not being assigned to a Group.
If you are using Groups within invitbox the users can only view invoices from Suppliers that are linked to the Groups that they are assigned to. If you have any Groups created, and a new User has not been assigned to any group/s they will not see any bills.
To learn more about Groups, please see here
Please note, Users with Admin privilege can view all Groups.