1. Prepare a CSV file with the following columns:

Contact Id - Ids of the contacts you wish to associate to multiple accounts.

Account Id - Ids of the accounts you'd like to associate to each contact row. Each contact / account relationship should be on a separate row. This means if you're associating 1 contact with 5 accounts, your spreadsheet should have 5 rows with the same contact id.

Roles - Optional. Available values can be found or set by navigating to Setup, enter Contact Roles on Accounts in the Quick Find box, then select Contact Roles on Accounts. You can specify multiple values by separating them with a semicolon. See Update multi-picklist field values using Data Loader for more details (Ex. "Business User;Decision Maker;Technical Buyer")

IsActive - Optional. Indicates whether relationship is active (true) or not (false).

Start Date - Optional. The date a relationship between a contact and account began. Used to keep a history of the relationship.

End Date - Optional. The date a relationship between a contact and account ended. Used to keep a history of the relationship.

Note: The format of the Start and End Date values in your file should be YYYY-MM-DD (Ex. "2017-12-01"). You may need to manually and explicitly set the format for each date column in Excel before saving your file. See Why do date fields import incorrectly when I use the Data Loader? for more details on how to format your data with: YYYY-MM-DD.

2. Populate the file with your desired contact and associated account record Ids as well as optional columns and their values and save.

An example CSV file, "AccountContactRelation Example.csv" is attached to this article to help you get started. NOTE: In the example file there is only 1 contact that's being associated to 3 different accounts for illustration purposes.

You can find existing Account and Contact record Ids by viewing records in Salesforce as outlined in the Unique Record IDs in Salesforce. Alternatively, Build a Report and Export a Report in Comma Delimited .csv format or alternatively use an API tool such as the Data Loader to Export Data by selecting to include Account and Contact Ids along with any other desired fields to identify your records.

3. Once satisfied with your prepared file, open Data Loader and select:

A) Insert | Login | Next 

B) ​Click "Show all Salesforce objects" to locate and select "Account Contact Relationship (AccountContactRelation)" from the list. Click the "Browse..." button and choose the csv file you've created | Open | Next

C) Verify that the number of records in the prompt is correct and click OK. Click the "Create or Edit a Map" and then "Auto-Match Fields to Columns". Verify that the columns are appropriately matched to the Salesforce fields and click OK | Next 

D) Click Browse... and select a location on your local machine for the resulting success and error files. Click Finish | Yes.

See Also :
Considerations for Relating a Contact to Multiple Accounts