What you need is to run some SQL on the iidbprivileges catalog in iidbdb. Check out appendix A of the database administrators guide for more details as there are lots of things you can find out, but the specific query to answer your question would be something like
Code:
SELECT grantee_name FROM iidbprivileges
WHERE database_name = '<your_database>'
AND gr_type = 'U'
AND db_access = 'Y'
You might also want to consider that some databases are public, which means everyone has access by default. You can find this information out by looking at the "access" column in iidatabase_info (also described in appendix A).