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 on this here .
Once an admin has the proper permissions, adding visitors/guest to a booking is a rather simple task.
First step would be to navigate to the booking you want to edit. That can be done either directly through the Calendar or via Community>Bookings.
When you click on the booking, a window with its properties will open. There you'll be able to see a checkbox called "Invite guests". Please 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.