One of my guiapigs keeps bitting the other ones ears and face

It could be a territorial issue. Most of the time Guineas get along fine if they are raised together no matter the sex, but if you have 2 males together of different ages (and sometimes even females) then it can result in fights.

You may need to separate them or try a larger cage where they can have time away from each other and have their own beds in different sections.