"What is it?" GroupMe is a cross-platform mobile and web messaging app which allows you to create/join groups or talk to any selected individuals whenever you like. More information about the app can be found on its official page here --> GroupMe
"What's its purpose within [F9G]?" The app provides means in which you can communicate with clan members instantly, whether you want to setup parties for COD, Destiny or just keeping up to speed with clan events. Send a message within the according chat of what you want to do and have clan/squadmates instantly reply to you. People can use it while nowhere near their home or game system to plan ahead and dominate the competition.
*Note: Notifications of incoming messages as well as any sounds/flashes/popups etc... can be turned off and regulated as you like through the app itself. It is very customizable and easy to use.
"How do I join?" You will recieve an email(at the address you registered with) with a link to join your respective groups. If you do not recieve the email or are having any problems please go to the contact page on the website and inform the admins.
"Is it mandatory?" Yes, but if you wish to stay up-to-date, have instant communication and get to know your clan and squadmates it is highly recommended you join.