This article details the steps taken by System Admins to enable the Grouped Occurrences.

NOTE: While the ability to group occurrences makes it easier for volunteers to sign up for a series, there are business practices to be considered . System Admins should only enable these features for trained users. It is not recommended that you enable this feature for Partner Staff. If desired you can use permission sets to enable it for designated staff and partner staff users. (Grouped occurrences cannot be enabled for basic portal partner staff).

To learn more about permission sets, see this Salesforce post