How do I add an email to Salesforce when it has multiple contacts/recipients?

When logging an email with multiple contacts, Cirrus Insight will automatically relate the email to all the recipients' Salesforce contact records; your org just needs to have Shared Activities enabled in Salesforce.

You can confirm that the email will be related to more than one contact record by clicking the Add to Salesforce button from the email. You should see a contact name and then a + #, as shown below:

If you don't see this and your emails are only being related to one contact, reach out to your Salesforce Admin who can enable Shared Activities: Salesforce Help - Enable Shared Activities.

After Shared Activities is enabled, refresh Cirrus Insight (How do I refresh Cirrus Insight?).