Chelsea, fractions can be confusing, but let's do this.
So, 3 of 4 parts of the club is 12 people. If we break 12 into 3 groups, we get 4 members in each group, meaning that 1 of the 4 parts of the club is 4 people.
Since we have 3/4 present, we are missing 1/4, or 4 people, meaning there are 16 total members.
To do this algebraically, the word "of" means to multiply. If x = the total number of members, then 3/4x = 12. Diving both sides by 3/4, we get x = 16.
Hope this helps!