Hi Euan,
First of all, in order to get the current user’s email you have to ensure that there is a member who is logged to your site.
You can use the built in member system.
https://support.wix.com/en/article/adding-a-member-login-button-4628013
Now you can use the function getEmail():
let user = wixUsers.currentUser;
let userEmail;
user.getEmail()
.then( (email) => {
userEmail = email;
} );
Pay attention!
create the variable “userEmail” at the global scope in order to use it outside the function’s scope.
Now, in order to add the “userEmail” field to the collection, use the function insert().
You can read about the function in the link below:
https://www.wix.com/code/reference/wix-data.html#insert
Have a nice day and best of luck!