There are a gazillion threads on this topic. Do a tag search for "metamour" or "metamours" to find them. Any partner of someone you would be dating is considered your metamour in poly-speak. Lots of threads here about managing that relationship.
And no, it isn't necessary to be friends with a metamour, but most folks generally feel that it's good to at least be acquainted and have a respectful polite acknowledgment of each other. But there are people whose metamours never even meet (although they should know about each other, for honesty's sake) and everything works just fine, so it's up to the parties involved.
The world opens up... when you do.
Oh, oh, can't you see? Love is the drug for me. ~Bryan Ferry
"Love is that condition in which another person's happiness is essential to your own." ~Robert Heinlein