Head to the People's page under your Brokerage Menu

Click the 'New' button

Fill in the contact's information

The fields highlighted in yellow are required

The fields with blue borders are required if the contact is an Agent, contact that's paid to an organization (company) name, learn more about organizations

Fill out any further fields below and click 'Save' to update.  If you're adding an agent, click here to learn more about agent contacts. 
All contacts including BUYER or SELLER, except Organizations, can be added on the fly within a deal, 

How to add People in a deal: https://support.loft47.com/portal/en/kb/articles/adding-people-to-a-deal
How to add BUYER/SELLER in Loft or in a deal: https://support.loft47.com/portal/en/kb/articles/adding-buyer-se

Here's a quick overview on adding contacts in Loft