After creating users and shares you can assign users to the shares.

When adding users to a share, go to the share and search for the user on the list, 

choose R (read) or W (write) access to the share for the user.

If you need to remove the share from a user again, you click on the "remove link"

When adding new shares to the user, the system will send out an information mail for each

share the user get access to.

If you need to change the rights for a user you can directly on the list change between

R (read) and W (write) on a specific share.