cjdns does not exactly need an invite. If you want to peer over the internet you'll need to swap public keys with someone already part of the network. But most people have turned on autopeering, which will discover peers on the local network and take care of peer swapping for you. Keep in mind that the intended purpose of cjdns is not to be a p2p library, it is to be a mechanism for connecting local mesh networks together into a mesh internet.

cjdns dev here, the network has been opened a while ago. There are some public peers floating around.

There's nightfall[0] to find/announce public peers, but I consider it very beta (quickly hacked together on a bus trip).

[0]: https://github.com/kpcyrd/nightfall

cjdns can be used as a private mesh also so it completely depends on the application. A private mesh could be more appealing to some.