To generate a link that will allow anyone who follows it to join a discussion as a Writer, go to the discussion menu (click on the three horizontal bars at the top-left of the screen), and select ‘Discussion Settings’. Select the ‘Invites & Permissions’ tab. Under ‘Discussion Writer Invite Link’, click ‘Enable’.
Note that this link can be turned off at any time. You can only turn on this link in discussions that you’re an Admin or Owner for. If a user has turned off the ability for other users to invite them to discussions using their email address (see: Turning off Email Invites), they will not be able to successfully follow a discussion invite link.
Alternatively, you can invite new participants directly. For more information on this option, see Inviting Participants to a Discussion.