How would you treat a friend poacher, someone you barely know who invites your friends out & excludes you?  ?

If they barely knew me I wouldn't expect them to be inviting me out. That would just be weird. Your friends are allowed to have other friends too and most people don't expect to have to be saddled with a whole group if they really only want to see one or two other people. 

Like somebody my friends know. I don't have a "friend group" that associates only within themselves. I know we all have other friends we do things with. Honestly, I've never kept track of what everyone else is doing and with whom they are doing it!  

I wouldn't treat them in any way. I wouldn't deal with them.

Like anyone else. Yes, it hurts to be excluded when the rest of your normal social group gets together, but that doesn't require you to behave in any particular way. Are you seeking retribution? Since you don't "own" any of your friends, I don't see why you feel someone is poaching them --  taking or acquiring them in an unfair or clandestine way.

If you want to know why you weren't invited along with other people, ask the people who were invited.  Maybe they have another connection to the new person that you don't. Maybe the new person doesn't like you because you are suspicious and possessive. Maybe they were wrongly informed that you weren't available or wouldn't be interested.