These commands need to be run using sudo.

Export the username you want to add:

export NEWTRACUSER=nickname

To add a user:

trac-admin /web/tech.transitionnetwork.org/trac permission add $NEWTRACUSER admin

To set their password:

htpasswd /web/tech.transitionnetwork.org/.htpasswd $NEWTRACUSER 

You then need to login as the new user to set their email address for email notifications to work.