How can I join groups of blockchain developers? Where do they communicate?

You can join forums. There are either specific (for instance, http://forum.ethereum.org/) or general (Bitcointalk.org). These forums have categories devoted to development of blockchains of cryptocurrencies. You can make a first impression there and maybe join Slack or Telegram groups in future.
 
https://medium.com/blockchannel/where-can-i-join-the-ethereum-community-3aa5c795b1e5