The ONAP Slack instance is https://onapproject.slack.com/

The join link is https://join.slack.com/t/onapproject/signup  It should permit most company & school domains known to be engaged with ONAP to join without difficulty.

Folks joining from a "general use" email domain like gmail, hotmail, qq, etc. will need to ask a peer to send them an invite.   

Unfortunately there are is at least one vendor and a few carrier domains that Slack also throws into the "general use" category even though they have legit usage for employees within those companies.