Hello Tito Chen,
We just got a reply from our internal team about your current concern.
And these are the information they have provided:
Given that the MS Loop is still in preview, the option to change some permissions of the of your members might not be available for now.
That is why it is best that you send a feedback about this matter directly to our developers.
Here's how:
Want to give feedback?
During this preview we want to hear from you to help improve your experience – so send us your feedback in the app to help us shape the journey.
In the bottom right corner, select Help> Give feedback.

On Android or iOS, tap your profile picture at the top left to find Help and feedback.
On the other hand, you can also Discover how administrators can access assistance for a Microsoft 365 Business subscription by reaching out to our support team.
Referenced link: Get started with Microsoft Loop - Microsoft Support
Let me know if this helps,
Sincerely
Khou Sun
Microsoft Community Moderator