However one thing I don't like is the fact, that so far I couldn't find a way to "whitelist" those who can post on created by me community.

That will indeed be a problem. Once a group gains popularity and has a lot of subscribed reader there will be SPAMer to post into the group just to get more eyes onto their postings.

Of corse a while list would need to be build into the blockchain itself or SPAMer will just use an alternative interface or the STEEM API itself to post.

I have seen this happening both in GAB and Minds groups.

Using downvotes to solve all SPAM on the platform isn't enough.