Run trac-admin commands on the server hosting the trac sites. In our Debian server setup at least, individual projects are in the /srv/trac/ directory, and every trac-admin command should be followed by the path to the project you care about.
For more information than man will give you, use:
trac-admin help
To make a user an admin (I believe this will automatically create a new user if it has to) --
trac-admin /srv/trac/example permission add username TRAC_ADMIN
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