Vendors can register for an account at:
https://[your-shop-domain].myshopify.com/apps/vendor-portal/login/?register=1.
Once registered they will not see any data as you will need to connect a vendors data to this account.
Once registered you will need to connect a vendor for the new login to see any data. You can do this from the accounts page.
Once on the accounts page you will need to click "EDIT ACCOUNT" next to the new account you want to assign a vendor to.
In the edit account popup you will be able to search and assign multiple vendors for an account to switch between in the portal. You can also disable their account to prevent them from logging in if you need.
Once vendors are assigned, simply click save and that login account will now be able to see the assigned vendor data.
If the vendor does not yet exist to assign, please proceed to make that vendor.