כיצד להגדיר את Discourse לקהילה סגורה או פרטית

How do trusted users or staff invite new users? Do they walk up to their friends and say ‘hey, why don’t you join our discourse forum!’ If so, how is the invitee to go about it?
Or are new users invited by email with a signup link? Are the invitation emails sent out from within Discourse? If so, how is that done?

I’m just curious to know the method or process of ‘explicit invitation’.