You almost certainly don't need to create such a user.
Samba (in Ubuntu and in other operating systems) can be configured to provide resources, including shared folders, to anonymous users.
For example, to share a folder in Nautilus (the file browser) via Samba so that it's accessible over the network by anyone, without logging in, right-click on the folder, click Sharing Options, and check the box labeled
Guest access (for people without a user account).
This is just one example (of the most common sort of situation where one may want to enable guest access). If you need information about how to configure Samba for guest access manually (by editing configuration files) or for other resources (besides a shared folder), you can edit your question to provide details.