This is story related and literally impossible to miss as Selma blocks your way. Simply approach her, watch the cutscene and she’ll have joined your crew by the end and your achievement will have popped (the achievement makes it sound like you have to do something, but you don’t, just walk up to her).
Selma comes with an Ancient Pistol, your second Silent weapon, you don’t have to keep it on her if you don’t want, but it’s critical for game progress.
As this is your first additional companion note that she’ll have a bunch of mutation points to spend. Companions all earn mutation points whether they’re in your active party or not.