Hi @Dario Iacchelli ,
Yes, MS Planner plan works when the associated plan site has dynamic membership setup.
No license is required for devices that are members of a dynamic device group. So MS Planner plan user can become a members of a dynamic device group, then use Dynamic Groups to access SharePoint Online sites.
Reference:https://learn.microsoft.com/en-us/entra/identity/users/groups-dynamic-membership
https://blog.atwork.at/post/Dynamic-groups-accessing-SharePoint-Online
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