This problem should alleviate itself. If you add everyone in the list and someone adds you, and you hover on their profile and see that you have 5, or 10, or 15

This problem should alleviate itself. If you add everyone in the list and someone adds you, and you hover on their profile and see that you have 5, or 10, or 15 mutual connections (usually with foreign names), you can infer that it must be someone from HN that's adding you.