To maintain security I need to BCC my contact groups on desktop it is easy. Where are my contact groups on the mobile app? I need this functionality.
Based on your description, it looks like you are creating an M365 group in outlook? Create a group in Outlook - Microsoft Support. If that is the case, all the members will see each other. I'm afraid there's no way to hide every single one of them from each other. One work around I could think of is adding your customers as contacts in outlook and then creating a contact group for them. Create a contact group or distribution list in Outlook for PC - Microsoft Support. The customers cannot see or access this group as it is local to you. While sending the email, you can put the contact list in the BCC box such that they can't expand this to see the other members.
In case you have any further queries, please let me know.
Best Regards,
Ashraf.