Step 1: Prepare your data

In your Excel spreadsheet, create a table with two columns:

RelationId – Contains the ID of the person (User, Contact, or Lead) or the resource invited to an event. When Shared Activities is enabled, RelationId can also contain the ID of an Account, Opportunity, or other object related to an Event.

EventId – Contains the ID of the Event. 

IsIntitee – Set to True if assigning User, Contact, or Lead as an invitee.

Is What – Set to True if assigning Account, Opportunity, or other object related to an Event.

IsParent – Must be true if Is What is True.

Populate the table with corresponding Ids:

You can use the Context tab of XL-Connector 365 to lookup the required Ids without going into Salesforce to fetch them:

Step 2: Insert the records using the XL-Connector 365

1. Select the rows with invitee Ids that you want to create and click on Create/Modify ->Insert Selected:
   
2. In the subsequent screen, specify the object to insert as EventRelation:
3. Click on Mappings and make sure the two columns are mapped to the correct fields in Salesforce and the boxes to enable the mappings are checked:
4. Save the mappings. You'll return to the previous screen where you can now run the mass Insert operation by clicking on Run and Save:
   

Once the mass insert completes, the operation results will be placed in the same sheet, on the right from the data, one column over: