That triggers available variables are as follows.
Code:
$_PLUGINS->trigger( 'onAfterAddConnection', array( $this->referenceId, $connectionId, $ueConfig['useMutualConnections'], $ueConfig['autoAddConnections'] ) );
You won't have access to the recipient users email address from this unless you change the User for the auto action. With the above trigger var1 is your user id and var2 is the user id you're connecting to. For example the below should work.
User: Manually
User Variable: Variable 2
Now substitutions should be based off the user you're connecting to. Next if you need to substitute in something from your own user you can just use the user="#me" usage on your substitution. Example as follows.
[username] = Connections Username
[cb:userdata field="username" user="#me" /] = Your Username