In Facebook, how can I restrict my own posts in a group to only be visible to the group members? In other words, I don't like the posts that are only relevant to the members of that group being visible to my friends who are not in that group.
Suppose your group name is "College Friends". Simply goto Browse Groups By Membership and select that group and post it there.
Another way is by creating a list. To do that just goto here and click create a new list. You can suppose name it "Relevant People" and add the people in that "College Friends" Group. Then make a regular post and set the privacy to that newly created list "Relevant People".
That way you can make a regular/personal status/post that will only be visible to people in "College Friends" Group.