Yes, you can add the same email address to the Details section of multiple Client records.
If Client A has ClientA@example.com on file and you add that same email address to Client B's record, you'll be notified that you're adding an existing email and asked whether or not you want to proceed.
If you proceed, all email communication (Booking Confirmations, Intake Forms, etc.) for both Client A and B will be sent to ClientA@example.com.
If a MassageBook Client account is created under that email address, activity for both A and B will be available for viewing and managing there.
To learn more about how a Client shares an account and books online, go here.
Additionally, Gmail addresses have a capability that allow you to create different MassageBook accounts under the same Gmail account.
Let's say a husband and wife share firstname.lastname@example.org. The wife can use email@example.com for her own record and Client account.
The husband can add a "+" to the end of family, followed by whatever else he chooses. For example, firstname.lastname@example.org or email@example.com. Either of those will be recognized as a different email address by MassageBook, but all email communication will still be delivered to firstname.lastname@example.org.