I installed Edgewall Trac0.12.3 and added myself to the administrators group. In the administration panel I see the following:
AFAICR "Users" group must be available here. By using this group, I could add a user, manage his rights, specify mail and so on.
Maybe I need to install some plugin?
On a base installation without any plugins, you can only manage user permissions from the web interface (Permissions option on your screenshot).
To manage users from the web interface you'll need a 3rd party plug-in like, for example, the Account Manager Plugin (there others, so I suggest you first do a survey of which one suits your needs best at Track Hacks).