As the example shows, the users to be added to the Information Barrier group (specified as a list of user IDs) must be included in the data parameter.
Roles and Privileges
Calling this endpoint requires the ACCESS_USER_PROVISIONING_API privilege.
See Bot Permissions for a list of roles and associated privileges.
Note: The underlying call to add the user ID is synchronous, adding a large number of ID to an Information Barrier group may result in a long wait for this call to return.
The maximum number of user IDs that can be passed in one call is conditioned by the size of that parameter: Maximum 2Mb.