The singular guy does but the plural refers to a group of people. The group can be all girls, all guys or a mix and it is used casually.

'Guy' in the singular indicates an adult male. For example, if you heard someone say 'My boss is a nice guy', you would know that this person's boss was a man. 'Guys' in the plural can refer to a mixed group. In an informal situation, you could say 'Hi guys' to a group of people including men and women.