Map groups are a way to group markers together, where showing or hiding a group on a map will also show or hide all markers in that group.
For example, you can have a group called Settlements, one called Forests and one called Ruins, and place all of your map markers in those groups. When viewing a map, you can toggle the markers in those groups with a simple click.
Groups have the standard visibility permission system, meaning you can make group only visible to your campaign admins, or only to yourself and hidden from the other admins of your campaign.
Standard campaigns can have a maximum of one (1) group per map, while boosted campaigns can have up to ten (10).
There is currently no way to have more than ten groups per map.