@Chris Mancey
First, you should enable external sharing both in the tenant level and site level.
(1)Enable external sharing in the tenant level. Go to SharePoint admin center -> Polices -> Sharing -> Set Anyone in the Content can be shared with section.
(2)Enable external sharing in the tenant site level. Go to SharePoint admin center -> Sites -> Active sites -> Select the site -> Sharing -> Set Anyone in the External shaing.
Second, go to the Task list -> List settings -> Permissions for this list -> Stop inheriting permissions -> Grant permissions -> Enter external users emails to grant permissions for them.
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