To enter a shared contact in the Web portal, you must be an Office Manager in the system.  

From the Home Tab, click on Users.

When in the Users Tab, click on the Shared Contacts Button

Next, click on the Add Shared Contact button

When you have entered the contact's information, you can click on save.  If you have more to enter, click on the up arrow next to "Save" and it will show "Save and add another".