A group is a collection of users with a shared set of permissions.

You could manually assign these permissions to each user, but that would be a lot of work! Instead, you can put these users in groups and assign specific permission levels to the group, and all users now have the same permissions. A common example of groups is used in Slack. In this case, the resource is a specific channel. You can create custom groups that can be added to different channels. For example, you can create an Engineering or Product group and add the entire list instantaneously to a new channel you have created. At its most basic level, a group is a user list. You can share these lists with other systems so that you can preserve group structure easily across different systems in your organization.

Typically, groups are associated with the organizational units at a company. For example, you might have an Engineering group with shared permissions, or a Sales group with shared permissions. A group is not just an arbitrary bundle of permissions; rather, it implies information about what a member of the group should have access to, i.e., sales people should have Sales permissions, and engineers should have Engineering permissions. This is distinct from roles, which are focused more on dynamic job functions at a company.