Girls is the female form of ‘guys’.
Girls is female form of 'boys', clearly so. And you wouldn't generally address a mixed group of males as 'boys'. If they're your friends you might say 'my boys', and if you're deliberately trying to create an overly informal atmosphere you might say 'hey boys', or as a kind of flirty greeting from a girl 'hey boy', but you wouldn't walk into a room and say 'hey boys' - but you could and would with 'guys'. Think about it in professional settings, too. You could certainly say 'what's on the agenda guys?' in meeting, for example. You wouldn't use girls/boys in that setting.
As a result of that, I'd say guys is increasingly used as a gender-neutral term, precisely ***because*** the language lacks an informal term for a group of women that doesn't come with certain connotations. Saying 'girls' to a group of women is clearly diminuative, it has an implicit lack of respect or over familiarity because it's obviously younger than women are. Same as using 'boys' would for men in the wrong context.
Saying 'guys' doesn't have any age implications, it has connotations of friendliness and informal grouping.
What else could we use for women that does the same thing?
Ladies? No.
Sisters? No.
Lasses? For me yes, lads/lasses had the same connotations for me, but it's not used throughout the UK and it's too informal to be useful in all settings, EG with a new group or in a professional setting.
Women? We just don't use 'women' or 'men' in that way for some reason.
So, what's happening? Guys absolutely gets used for a group of men (of mixed ages too), for a mixed group, and, increasingly, I find I'm using it for groups of women too, with no-one batting an eyelid.
Possibly also helped by there being a direct Italian equivalent in 'Ragazzi' - it means there's just a few more English users to latch onto that use of 'guys' and spread it around even more.
