Servers bring invite only is like when Hey was invite only. It doesn’t mean you can only talk to other Hey users it just means being hosted by them is exclusive. The only thing you miss out on is the feed of what people on your server are posting -- maybe that's not a problem or maybe that's the raison d'être.
You can use a service; not all of them are invite only, or you can setup your own blog software. Federation is pretty close to RSS. Yes it's trying to be microblogging.
Is Facebook a microblogging platform too?