As ever with these things context is important, as is whether or not there is any existing familiarity, and the extent to which local colloquial terms are the norm.
Rhi has mentioned 'darlin(g)' and 'mate', both of which would I think come across oddly here in Glasgow, where the former feels patronising to me because, I suppose, it would be unusual here whereas addressing a male as 'Jim' or 'Jimmy' or a woman or girl as 'hen' would be unremarkable.
A phrase like 'how are you guys doing' as a collective term is one I've used but only where it is someone I've had a conversation or e-mail exchanges with previously: to use it with reference to people some/all of whom I'd just met, such as in a formal meeting (in the bad old days when I had to earn a living), and where I didn't really 'know' at least some of them, would seem crass to me. Perhaps some people feel that a more relaxed or laid-back approach when addressing people is better: whether those being addressed would agree is another matter.
I latterly worked with a guy, a fairly senior manager, who invariably used to address mixed gender groups as 'chaps and chapesses', which made everyone cringe!
