It's pretty easy to forget what groups you've added a contact to, especially if you have a lot of contacts and a lot of groups. But there's a simple way to figure it out. All you've gotta do first is select the contact card in question:
Then just hold down Option, and the groups that person is a member of will be highlighted in blue in the sidebar.
Before…
…and after.
Neat! And look, this tip hasn't yet turned into one of my incredibly long-winded, overdone, too-silly ones! Must…resist…urge…to keep typing…things…
Whew. That took a lot of effort, people.