Additional considerations
If the intent is to create a community as read only (that is, only admins can post), the community should be created by the corporate persona account.
This is the same account which is configured in the admin console settings. While site collection admins and users with full control can post directly to an admin only community, a limitation in the API only allows for the creator of the community to post.
A workaround would be to set the IsOwner flag to 1 in the SocialGroupMembership table.
Please consult Aurea Social before manually changing database values.