The Bulk Add Engage Points Express Action lets you choose attendees and add a goal (achieved) in bulk.
First, select the goal you want to add to everyone's record.
Choose a Target Group using the filter.
If you want to, you can choose to enable Push Notifications to send a notification to each attendee that they've 'achieved' this goal.
Select Start. Once started, the bulk assignment runs in the background.
You can also look under the Selected Records tab to individually de-select or re-select attendees from the list who'll receive this goal.