IN your models include

class IS_EMAIL_LIST(object):
    def __init__(self, error_message="Email %s is invalid", sep=","):
        self.error_message = error_message
        self.sep = sep

    def __call__(self, value):
            emails = value.strip().replace('\n','').replace('\t','').split(self.sep)
            for email in emails:
                email = email.strip()
                if IS_EMAIL()(email)[1] != None:
                    return (email, self.error_message % email)
            return (emails, None)

Now you can use it on tables you define:

db.define_table('invitation',
                Field('list','text', requires=IS_EMAIL_LIST())
                )

Users can pass in a text box:

"email1@gmail.com, email2@gmail.com, email3@g"

in the above case, last email adress will fire the validator to return an error message.

You can change the separator by defining the sep argument.