How do you follow someone and search them?

To search for someone, put this in the URL bar.

scratch.mit.edu/users/PERSON

Of course, replace PERSON with the user you are looking for.
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To follow a user, there will be a blue button in the upper-right hand corner of the page. It says “Follow”. Click it. If it's not showing up, then check to see if you have an ad-blocker. If you do, make sure to turn it off.