1. Does the setting GJ > Configuration > Notifications > Global > Notify by [PMS or Email, PMS & Email, PMS only, Email only] effect all GJ notifications? E.g. Message Members as well as Join / Leave / Invites / ....?
Yes.
2. If the setting is "Email or PMS" where does the user select his preference?
They don't. If PMS fails it trys to send email.
3. Under GJ > Configuration > Notifications > General > Display there is a text field with a description "Optionally add description to be shown above general notifications display." Where does this description show up on the front - end?
On the notifications page when you navigate to it from any of the menus (overview, category, or group views).