This is not possible because anyone submitting content to K2 must have a valid Joomla user ID.

If it's moderation that you're after, you can always configure one or more K2 User Groups (for frontend editing) that don't have the rights to "publish" content.

If you're looking for easy signups, you can always check out our own SocialConnect which significantly speeds up user signup by allowing them to login using the social network or other identity provider account (e.g. Facebook, Twitter, Google, Microsoft and many more): www.joomlaworks.net/extensions/commercial/socialconnect

This is actually the Users component in Joomla and your template provider *should* include appropriate styling.