
Guest users will be simply included in the MAU billing for the Entra tenant. The pricing is listed here: https://azure.microsoft.com/en-gb/pricing/details/active-directory-external-identities. This is only charged when the tenant has more than 50000 MAU which is any user that signs in during a calendar month. I imagine that you are far away from this limit.
You will only be charged if you actively assign licences to the user like to Business Basic licences you mention here. This does mean that the user will not have access to many Microsoft 365 features such as Word, Outlook, Sharepoint etc etc but by the sounds of it this is acceptable.
I hope this clears things up for you.
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