If you were greeting a mixed-gender group, what would have been the most common way to do this in classical era Rome? Would they have said something like salvete amici et amicae, or would they have only used the masculine noun?
In Classical Latin, like in many of the older Indo-European languages, the masculine was very much a "default" gender. (The feminine was a relatively late development within Proto-Indo-European, and it took a few more millennia for it to become as common and well-established as the masculine.) So a group of friends of mixed gender would be, by default, amīcī.
If you don't want to use a gendered form, though, you could also use a word like socius ("comrade"), which is epicene—it can refer to people of any gender, but is always grammatically masculine. So a group of comrades, no matter what their genders might be, would always be sociī.