Go to the calendar and click on the edit or register button:
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Make sure that you are on the bookings tab and click on "Add booking"

Search for the person you want to book's name and select them from the dropdown list:

If you can't find them on the list, you will need to first add them to Plinth. To do that go to "People" on the left handside and then "Add new members" in the top right corner. You can then select a form and add their details.

Check their details on the form and then click "Save".

Choose the dates that you want to book them on and then click "Book".