The first book I read on game was The Mystery Method which stated that you shouldn't open the target but NEG her until she starts to show IOIs.

Currently I am reading Jugglers Method and just came across this quote:

In most instances it is best to approach one (your target)and then quickly include the others as they give you their attention. This will put you next to your target and yet polite to the people in her group. After the girl has introduced you to her friends and you can either talk to them some more or resume one on one with your target

So he advises to approach the target and then include the rest of the group.

Opinions on this from experience in the field? Which method works better, or does it depend on the PUAs style whether or not the target is a HB9 or HB10 and needs to be NEG'd, etc.

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Life begins outside your comfort zone.

There is no one answer to this. I've done both and have had success with both. Some will open up with one way, some won't, others will open up with the other way, and still others wont. These are social dynamics, they are dynamic.

There is no definite answer to that. If your target is looking for someone (e.g. she is single), you match her taste in looks, and she is mature enough to be independent from the rest of her group, you will definitely be able to open her first. In other cases, opening any girl, no matter whether she is your target, the reaction may be the same.

The safest but not always the most efficient way to approach is indirect. Talk to everyone in whatever order feels natural, preferrably the men first. That way you give them the impression of "just being a nice and funny guy" instead of a "try-hard who wants the girl". You also give your target a chance to start liking you before you start talking to her.