How can I add my own fields for every client (e.g. to mark they signed a contract)?

Adding customized fields

Go to Settings, General and in Custom field settings section click Additional client-related fields.

Click on Add field and choose the type of field you want.

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Changing field for the certain participant

Go to Participants and choose certain person. Find Custom fields section and change the information you are interested in.

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Summary preview
To see a summary of your own fields for all customers, go to the Participants tab and click Show other fields.

Screen Recording 2021-04-22 at PM