Do you want to keep track of which diary users are making/editing bookings? By setting up a security policy you can choose when users require a password to complete a task.
How to set up a security policy
Go to settings - security and user permissions - security policy
If you want to have a username required when booking select the option 'Force Username Only', This means you would only have to enter the username when making a booking.
Top Tip - If you force userame and password you can reduce the password complexity to 4 characters which will save time completeing the booking (minimum requirement is upper case, lower case, number, for example Ab12}
The username and password credentials are added in the bottom left of the booking form screen: