到底是a member in the xxx club,还是a member of the xxx club?

到底是a member in the xxx club,还是a member of the xxx club?
题目是这样的：She's a member () the swimming club.
一个选项是in,还有一个是of,我选了of,但是老师为什么说应该是in呢?之前都是of的,为什么变成in了呢?