Thank you for posting this in Microsoft Q&A.
I understand that you are looking for a way to disable the "Welcome" email for M365 Dynamic user security group in Entra ID or EXO.
Unfortunately, there is no cmdlet or solution to disable the Welcome email for M365 Dynamic user security group in Entra ID or EXO.
Feel free to express your feedback regarding the "Welcome email for M365 Dynamic user security group" in Entra on our feedback Portal https://feedback.azure.com/d365community/forum/22920db1-ad25-ec11-b6e6-000d3a4f0789 All input shared in these forums undergoes monitoring and review by the Microsoft engineering teams responsible for developing Azure.
After creating a group, every group gets its ObjectId as its GroupId.
Hope this helps. Do let us know if you any further queries.
Thanks,
Navya