The first requirement to be able to add visitors/guests as an admin would be to have the proper level of access. You can read more about this here.
Note: Inviting a guest to a booking will grant them door access, based on the meeting room mappings that you have in your door access integration. More on door access integrations here.
Adding a Visitor to a Booking
Navigate to the booking you want to edit. That can be done either directly through the Calendar or via Community/Bookings.
Click on the booking, a window with its properties will open. There you'll be able to see a checkbox called "Invite guests". Note that this checkbox will only be available if a member is selected as a creator of the booking. If only a company is selected, the option will be greyed out.
If the checkbox is marked, a drop-down menu will appear, in which you'll be able to choose who to invite. This menu will contain all other members of the company that the member creating the bookings belongs to.
Below the "Guests" menu, there is a blue letter option called "Add guests".
This option allows you to add people that are not members of the company or the co-working space. You will have to enter a name and an email for the person. Guests added in such manner will be added to the Community/Visitors tab.