In Salesforce shared activities are used to let users relate multiple contacts to each event or task and to view information about open activities and activity history on contact detail pages. Whether you enable Shared Activities or leave it disabled, review these considerations.

https://help.salesforce.com/articleView?id=activities_shared_considerations.htm&type=0

To enable Shared Activities login to Salesforce.com and navigate to Setup > Build > Customize > Activities > Activity Settings



Set the Allow Users to Relate Multiple Contacts to Tasks and Events checkbox and press Submit button.