To get the invite link you would want to go to the Gather Icon in the upper left, or the participants tab in the bottom left, while in the space. Both of these options will bring you to a menu that will say "Invite" and once you click that you'll be given two links you can share. You can distribute this link however you would like, but email is most common.
Please note that you are welcome to send out the link in advance if you'd like. Your space will always be hosted on a free server outside of the paid premium reservation time, so up to 25 users can enter for free! If more than 25 users enter before your event, the server connection will degrade but you will not be charged.