Terraform Cloud's user objects do not contain any identifying information about a user, other than their Terraform Cloud username and avatar image; they are intended for displaying names and avatars in contexts that refer to a user by ID, like lists of team members or the details of a run. Most of these contexts can already include user objects via an ?include parameter, so you shouldn't usually need to make a separate call to this endpoint.
To find the ID that corresponds to a given username, you can request a team object for a team that user belongs to, specify ?include=users in the request, and look for the user's name in the included list of user objects.