Hi @Sarah Kathriner,
I'm afraid it is impossible to restrict site creation to specific users/groups. With the new admin center, you can only turn off "site creation" for all or none.
I would suggest you raise a thread in this feedback portal: https://feedbackportal.microsoft.com/feedback/forum/06735c62-321c-ec11-b6e7-0022481f8472. Our product team actively monitors threads there and will take your idea into consideration for future development.
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