OK, I saw that I cannot add tabs, but can edit them (thanks John DaSilva
for pointing me to the right direction in that one).

How about the "Categories" (not sure if that's the right term).

On the left hand side of the screen, a normal user see:

Information Management
Password Management

I, as admin, see that these are the categories to which I can assign
shared pages. The categories I see that are avaiable are:
Administrat, Directory Management, General, Guest Pages, Information
Management, and Password Management.

Can I add more categories? If so, is that controlled through a portlet
or somewhere else. If a portlet, which one; if something else, what?

Again, thanks in advance. I really appreciate the help I'm getting here.