document the difference between userid and username, and give
examples of circumstances where they might differ

fix typo in parameter designation
document that the method may raise UserNotFoundError
ensure that the username passed is treated exclusively as username, not
aliased to userid

raise UserNotFoundError if the provided username cannot get a user

document that this error may be raised
treat username as username only, do not alias to userid

raise UserNotFoundError if user cannot be found by username

document that this error may be raised
ensure that the non-existent user does not raise the wrong error
fix test to catch the correct problem

fix test to check for the right exception type
add test that removing a user from a group raises the appropriate error in
the case that the user does not exist