How can I search for a user in Adobe Connect using the API?

You can search for a user (principal) by email using the following query string:

action=principal-list&session=[BREEZESESSION]&filter-out-type=everyone&filter-email=[email]

Note that if you don't specify "everyone" than the query doesn't search through guest accounts.  This also assumes you have logged in and have a BREEZESESSION id.