If you know someone else who could use Client Diary, you can refer them from your own account.
Open the Dashboard. Under My Achievements, choose View More, then View My Referral Code. Generate a code. You get a link you can share by email, Facebook, or a message.
When they sign up with your code and become a paying customer, you receive one month of Client Diary free. They receive a free month as well.
The short guide is here: How to refer Client Diary.