With an Azure Resource Group Owner I could not find any user from the list of users in Role assignments. I got a message "An error occurred. Please try again later." instead. Please see screenshot here. screenshot The Resource Group Owner is a guest member. He is only owner of the resource group in question.
If you have restricted Guest access, they cannot enumerate users, i.e. they cannot get a user list from the AAD tenant. The user might be able to find another user if they type in their full username in the search field.
