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Global Address List (GAL)
The Global Address List (GAL) is the address look up feature in Office 365. It contains email addresses for individuals and resources, excluding those that are hidden. Office 365 uses the Global Address List to supply email addresses when you are composing a message or inviting individuals to meetings.

Group
Groups in Office 365 let you choose a set of people that you wish to collaborate with and easily set up a collection of resources for those people to share - such as emails, files, and calendar data. You don’t have to worry about manually assigning permissions to all those resources because adding members to the group automatically gives them the permissions they need to the tools your group provides.