Add multiple contacts to tasks with Data Loader

Warning : Always back up your data before performing any data operation. See Export Backup Data from Salesforce or Export Data for more details. It is recommended to run a test with a small subset of records to ensure the operation was successful through manually opening and verifying that the corresponding records are correct in Salesforce.

Note: The task records need to already exist in the org in order to follow this process. If the task records don't exist in your org yet, please refer the article on Import Tasks or Events (Activity History) into existing records and create Tasks using Data Loader. Leave the WhoID (Name) field blank during the initial task creation process. 

Now create a separate csv sheet with columns: TaskID, IsWhat, and RelationID

TaskID - The ID of the task to be associated with multiple contacts
IsWhat - This is set to False since we are setting a WhoID field
RelationID - The ID of the contact to be associated

Populate an individual row for each task / contact combination. So, if a task is associated with 2 contacts, that task should show in 2 rows. 
 

Once the CSV file is complete and saved, use Data Loader to upload the file to the TaskRelation object:

1. Open Data Loader and choose insert operation
2. Select "Show All Salesforce Objects" and select the "TaskRelation" object. 
3. Choose the above created csv sheet and map the three fields TaskID, IsWhat and RelationID correctly.
4. Click on Finish.