Collecting email addresses for a restaurant
In order to collect email addresses you need to offer customers some sort of incentive. This can take many forms and can be done inexpensively. For example you could offer a free dessert or a 10% discount or a complimentary glass of wine upon arrival. There are many options.
This incentive can be placed on your website for customers to see when they are visiting. It may give that extra push to book if they are unsure. It is easy to collect the address and direct them to a special page where they can collect a voucher or a code to secure whatever incentive you offer.
For existing customers who have visited you, ask them for their email when they pay and give them a free gift of something. Though most people, if they have enjoyed their meal will gladly give their email without an incentive anyway.