Passing a parameter through the URL

I have a site that lets user register then adds them to a db but they are not active.

Then an email is sent to the admin who then goes to an asp.net page and activates the user if he chooses to.

My question is how can I pass the UserID in that email link so that when the admin goes to the generic 'Activate User' page the info for the user at that UserID is brought up?

Basically I want to pass the UID in the email URL link and then grab it when that page is hit later.Registerthem.aspx?UID=blahblahbla

then on Registerthem.aspx:

string UserID=Request("UID");