A few people have asked us about this, and here's how it works at present.
When you join Typetrigger, you are automatically Reading the person who invited you, and when you invite people, they will be Reading you. To find the friends you invited, look on your Readers list (since they are automatically reading you).
If you know that a friend has joined but you did not invite them (and were not invited by them), you can search for their user name in the search bar at the top of the site. Please note that users will not appear in search until they have published their first trigger response. We will consider making changes to this, but we really do want people to write!
Finally, some people have inquired about an email friend finder, as is included in many social sites. We know that many Typetrigger users want to write anonymously, and being found via their email would spoil their cover. For now we encourage users to share their usernames with friends outside of Typetrigger and find one another by writing and searching. We look forward to hearing more user thoughts on this problem of sociability and anonymity.