Electrons themselves have a negative charge.
When you add an electron to an atom that is neutral, the atom becomes negatively charged.
Take Sodium (Na) for example. It has 11 (positive) protons and 11 (negative) electrons - it's neutral.
HOWEVER
If you add an electron to an already positive ion, then it will either stay positive or will become neutral.
An already negatively charged ion will stay negative when an electron is added.
This all depends however, on what the charge is already and how many electrons are being added.
For Example: If the charge is +1 and the ion gains 2 electrons, then the charge will then be -1.