I'm not sure if this is the same question as here:
https://forums.zotero.org/discussion/31969/collection-in-a-group-library-that-cannot-be-edited/#Item_0 (in that case, please don't double post), but you can't set separate permissions for individual collections. As the group owner you can assign members to user groups (members or admins) and give those respective privileges - including not allowing any editing by members - under group settings on this page.
https://forums.zotero.org/discussion/31969/collection-in-a-group-library-that-cannot-be-edited/#Item_0 (in that case, please don't double post), but you can't set separate permissions for individual collections. As the group owner you can assign members to user groups (members or admins) and give those respective privileges - including not allowing any editing by members - under group settings on this page.