When creating campaign members in Salesforce, you must link them to both a campaign and a contact or, a campaign and a lead. To do this, you will need to map those respective IDs in your connector.
If you do not map a campaign ID and a contact ID/lead ID you are likely to see this error message:
Attempted to add a campaign member where either the member id 'null' or the campaign id 'XXXXXXXXXXXX' is null.
To resolve this issue, ensure that you have mapped a Campaign ID and either a Contact ID or a Lead ID in your connector.