Is there a way to autojoin community chat when I log in?
No. I'm part of multiple communities. Sometimes I want to join a specific community at a certain time based on what I plan on doing. So if I want to rift, I join my rifting community. If I want to farm bounties, I join my split bounty community. Auto-joining one community for gear help or friends might come in the future, but there is no way of doing this as of yet.
EDIT: It has been updated and you now autojoin the community chat you were in when you left - WizLiz