Does Tettra support single-channel and multi-channel guests from Slack?
Unfortunately, adding integrations to Slack guest accounts is not currently supported since we are limited by Slack’s APIs.
Alternative
Slack has a Guest role feature that allows teams to collaborate with people outside of their company. You can invite multi-channel and single-channel guests to Tettra as Email users. Like other users, they'll be able to log in to Tettra with an email account and use the slash commands to find pages.
You can also set them as Guest users. However, they would only have access to specific categories, just as guests in Slack are limited to specific channels.