It is a recommended limit, not an enforced hard limit.
And this refers to the number of concurrent end users for a Cloud DataFusion instance at a given point of time. You can have more users sharing a given instance but only a fraction of them are using the instance at the same time.
To answer your questions.
* Yes. You can grant permissions to 50 or more individual users or groups.
* Yes. You can grant a single group with 50 or more members, but we recommend avoiding too many individual end users to use the instance at the same time.