After creating a new user, the user will have a unique ID called a "User ARN" account. It looks something like this for username called "admin":
arn:aws:iam:870464616830:/user/admin
The string of digits indicates the actual aws account.
IAM user's can log into their account using the AWS account's custom login url, which based on the above example will look something like this:
https://870464616830.signin.aws.amazon.com/console