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The first thing to do this is to copy the email address of the user and paste it into the search box on the people card. This will search over all the users in the system to see if any have that email address. This is a good start to checking if the user already exists, but isn't always perfect (e.g., some users might have multiple email addresses and may have registered using a different email address than the one that we had on file previously). After checking for that email address in the people card, repeat the same steps for the Potential Grantee's Full Name (First Name + Last Name) and then just their Last Name.
If you return no results after each one of these searches (i.e. the People card is as it appears in the screenshot, above), or the results returned do not match the Person in question, then you've confirmed the User does not currently exist in the system. If this is the case, follow the steps laid out in the "Creating a New User" section.