This phrase is informal and best used in casual conversations rather than formal settings. At the reunion, they gossiped about who had made it with whom back in college.
Pro Tip 2/3
Politely Indiscreet
The phrase is a subtle way to refer to sexual relationships without being too explicit. I heard through the grapevine that she had made it with him at the party.
Pro Tip 3/3
Slang Alert
Recognize it’s slang. Some people might find it inappropriate or offensive. Be careful when asking someone if they've ever made it with someone; it's quite forward.