The club has a female group which has twice as many members as the male group, so there are 2+1=<<2+1=3>>3 groups in the club.
There are 18 members in the club, which means there are 18/3 = <<18/3=6>>6 members per group.
Since the number of female members consists of two groups, then there are 6 * 2 = <<6*2=12>>12 female members in the club.
#### 12