How do I allow auditors to join the course's Slack workspace?

If the auditors have a current Harvard affiliation -- a current student, faculty, or staff member with an active student or staff role anywhere at Harvard -- then adding them as a Guest to the Canvas is sufficient to enable Slack access via the same method as a "true" enrollee.

If the auditors do not have a current Harvard affiliation -- such as someone from another university who isn't cross-registering, alumni, a former staff or faculty member -- then they will not be able to acquire "Full Member" status in Slack. You will need to invite them as a guest and be sure to add them to all channels you would like them to have access to -- guests do not have the privilege to add themselves to channels. See the Slack guides here on working with multi-channel guests:

• Understand guest roles in Slack | Slack
• Permissions by role in Slack | Slack

If the auditors were formerly in a Harvard Slack workspace as a full member (e.g. when they were an active student) then they cannot be invited as a guest using the email address that was used for their full membership. A personal email will suffice.