who get's to pick the new members?

Every house does it a little differently. At some houses the alumnae/chapter recruitment team has much more say than the actives in the chapter, while at others the entire chapter votes collectively or only the actives vote on the PNM's with whom they had conversations. Each chapter has their own method and none of them discuss it to outsiders. It doesn't matter to PNM's how it is done and isn't something to be concerned about.