Hi Erin. I expect that a SharePoint license is required. I know it's required for an Outlook group and for a Microsoft Team. Going forward, Yammer Files will become more integrated into the SharePoint document library. I don't there is provision for an Office 365 user in the same organization, without a SharePoint license, to take advantage of the SharePoint guest user access.
As Darrell mentioned - the key is to have 'a' SharePoint license - as long as there is one in the tenant, the functionality will be provisioned.
From a licensing perspective though the goal of course would be to have all users in compliance since they are using the functionality. I'd refer you to the TrueUp process and your Microsoft Acoount Team.