ArcGIS Online groups are a way to collaborate with other ArcGIS users. You join and create groups to exchange content related to a specific project or common activity. For example, you might be a biologist working on an international bird habitat map template. You might search for groups whose members are sharing local bird habitat data that you could use. You could join this group and share your template. Together, your group could create a standardized international habitat map with rich local data layers.
As a group owner, you decide if you want a public group that accepts member requests, a non-member public group (that shares the group content but doesn’t let other users join/share), or a private group where you, as the owner, invite people to join. For example, you might be a real estate investment firm who creates a private group for your research team. The team is working with proprietary data to map property values in a new redevelopment zone. You want the team to use the group to exchange their data layers with each other but you don’t want anybody outside your firm to find the group or its content.