Get the id for all registered users on team

I have created an internal message table for all users of my team.

Posting the each mail into a specific table view for each user and doing an auto table refresh at same time!

What I want to know is apart from getting my own ID value ie (current user me!)

Can i get the value for everyones elss ID on my team?

Furthermore, using an alert popup, is it possible that given an event that i can force a pop up alert for a specific user!

ie when the message has been set as URGENT etc.