"guys" is commonly used in North American English to refer to a group of people you're on familiar terms with. e.g., friends, you younger sister's or brother's friends, your close colleagues, be they all female, all male or a mixture of the two. "guys" is colloquial. For example, I have three sisters and no brothers, and I use "guys" to get my sisters' attention, as does my mom or dad when they want to get our attention; e.g., "Hey, guys, should we order pizza for dinner tonight?"
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