“Groups” give you a way to organize all your projects, fonts, templates, etc., under a single entity, so that you can easily share resources between different projects. For example, you might create a group for your imprint or company, so that all your projects would be created within the group umbrella. This would enable you to share design templates between all the projects in your imprint, bill them all to the same credit card (attached to the group owner’s account), share fonts between all the projects or users in your group, and more. To create a group, follow the steps here.
This video gives you an overview of groups, along with some of the benefits like shared billing information, shared fonts, and shared book design templates: