When a meeting is already scheduled you can invite an extra guest in two ways.
You can invite the guest by email.
Go to your Meetings overview page and go to the meeting you need to add the additional guest to. Click on the 3 circles in the upper right corner and click on 'invite new guest'. The guest will then receive a personal email confirmation with a link to enter the meeting.
You can also use this option if there was a spelling error in the email address when the meeting was created.
If you use the list overview instead of the blocks overview, the three circles are at the right next to 'Enter room'.
You can also share the link directly with the guest. Go to the meeting for which you'd like to invite an extra guest in the meeting overview. Click on 'Guest link' and all you have to do is paste it and share it along with the date and time.