The with function will let you use shorthand column references and sum will count TRUE results from the expression(s).
sum(with(aaa, sex==1 & group1==2))
## [1] 3
sum(with(aaa, sex==1 & group2=="A"))
## [1] 2
As @mnel pointed out, you can also do:
nrow(aaa[aaa$sex==1 & aaa$group1==2,])
## [1] 3
nrow(aaa[aaa$sex==1 & aaa$group2=="A",])
## [1] 2
The benefit of that is that you can do:
nrow(aaa)
## [1] 6
And, the behaviour matches Stata's count almost exactly (syntax notwithstanding).