Hi alexanderg,
Firstly, Power BI groups are built on groups in Office 365. You can create a group in Power BI, or in Office 365 -- it has the same characteristics either way. According to this blog, in Office 365, no limit is placed on the number of groups, groups with more than a 1000 members are supported, but group users may notice that accessing group conversations and the group calendar can take a long time.
Secondly, please note that pro users can create groups with a maximum 10 GB data capacity each in Power BI Service. Reference: https://powerbi.microsoft.com/en-us/documentation/powerbi-admin-manage-your-data-storage-in-power-bi... .
Thanks,
Lydia Zhang
Community Support Team _ Lydia Zhang
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