When you make a reservation you are going to want to link it to a customer record.
BRM lets you create a new client record 'on the fly' if you type in a new customer name when you're creating the reservation, or it lets you link a new reservation to an EXISTING client record (by selecting it from the drop-down as you type in your client name)
Once you complete the reservation (having chosen either an EXISTING client record, or creating a NEW client) you can then bring up the Client Details Dialog to enter in more details about that client.
You do this by pressing the 'client record' button which looks like this on your Reservation Detail panel:
(N.B. you can actually make this pop-up automatically when your reservation completes - its in your settings)
The Client Details Dialog looks like this. As you can see it lets you enter in user details like name, address, phone numbers and emails.
you may also want to incorporate automatic address lookup
See also re-connecting the client record