There are multiple ways to find users in your account.
In Global Navigation, click the Admin menu [1], then click the name of the account [2].
Locate search people on the sidebar. Type in the user's name [1].
After clicking Go, you will see the search results. To access that user's details, click the name link.
In Account Navigation, click the People link.
You can sift through all the users to find the one you are looking for by using the last name.