Can I activate her effect if I bring her out as my shadow card at the beginning of a phase or do I have to wait for an action window?

Her effect only works as a player action. You can bring her out of shadows during the shadows framework, but then you cannot trigger her effect.

Note that her effect is "bring Meera Reed out of Shadows" (the blanking is following a "then"). Obviously, you can't do that if she is already in play.