Google App Engine: Query users?

I am building an app that lets users sign up, and other users see data about those who have already registered. If I record the userID of everyone who signs up, how can I look up data about those userIDs later?

(I'm using the Java SDK.)

If you have signed-up users and data about them, having some sort of SignedUpUser Entity makes sense and is straightforward. From there it's a matter of arranging to construct indexes that support the types of lookups that you'll be doing (e.g., by name, by recency of activity). At this level, it's not much different from how you'd construct this on top of an RDBMS.