You can try the Flickr search. Use the box in the upper right corner, and select "Flickr members" as the search. If that doesn't work, you might try any email addresses you have for them, to see if they have set up a Flickr account using that email address. Other than that, you'll need to ask them their user name.

Meaning of Flickr in English the name of a website that allows users to share photographs and videos: You can see more of my photography on Flickr.