For groups created in ArcGIS Enterprise based off of an active directory group, there may be reason part way through a project where you need to invite additional users into the group from another department. Since they don't belong to the group in active directory, there's no way to easily add them.
For example, we are creating groups in Enterprise based on active directory groups that were setup around our org structure. If transportation is working on a capital delivery project, they may realize they need to invite a few users from environmental services into the project. Since the group was based on a transportation active directory group, there's currently no way to invite the env users into the group.
Even if there was support for multiple AD groups, there may be reasons why you wouldn't want to invite an additional full AD group into the group, just 1 or 2 people