In June 2018, we're making changes to the native social capabilities in SharePoint Online

bart_vermeersch

I checked with the Yammer team and this is what they say:

Technically, there are no limits to the number of members a group can have.
 
Groups 

•         A maximum of 100 users can be owners of a single group.
•         Any number of objects can be members of a single group in Azure Active Directory.
•         The number of members in a group you can synchronize from your on-premises Active Directory to Azure Active Directory is limited to 15K members, using Azure Active Directory Synchronization (DirSync).
•         The number of members in a group you can synchronize from your on-premises Active Directory to Azure Active Directory using Azure AD Connect is limited to 50K members.
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This is noted in the documentation here: https://docs.microsoft.com/en-us/azure/active-directory/active-directory-service-limits-restrictions
 
However, we do run into experience and latency issues in some cases with larger groups, however, 15K should be fine.