Finding a good time for a group hang can take lots of back-and-forth texting, so Doodle (free) is here for fast and easy scheduling in your Messages. Pick your availability and send it over to your friends to see if they’re available during those days, too. The best thing about Doodle is that your friends don’t have to download the app or sign up for an account to share their availability.
For more functionality, like setting up specific times and integrating with your calendar, try Kukutana (free). This iMessage app is more robust, however it does require your contacts to install the app in order to contribute their availability.