Just below the green header at the top of most forum screens you'll see a light-grey tool bar on the far right you'll see a link 'Members' if you click that it will bring up a list of members.
Unfortunately you don't have a search function for the member list. The best you can do is select from the letters across the table the one that is the first one in the individual's screen name that you're looking for. That will bring up a subset of all users that have the selected letter as the first letter in their screen name.
Down at the bottom of the page on the right side you'll find a page listing. the only thing you can do is make a guess as to which page the individual will be listed based on the spelling. If you over/under shoot you mark jump forward or backward as required, until you find it.
I just thought of it but a better solution right now in finding a member. Might be to use the Private Messages (PM) - Compose Message screen.
MLS menu bar > Community menu > Private Messages > Compose Message
In the Send To User field type in what you think the user name, then on the far right of that field you'll see a magnifying glass icon (search/look-up) click that and it should pull a list that closely matches what you typed in. That may be faster to find what you're looking for.