Both the miniExtensions Form and Portal extensions give you the option to have your users pick login details from a list, instead of entering data manually. This can be useful when privacy isn't a main concern, or when using the login more like a record lookup (e.g. look up a product record from a catalog).
Here's how to set this up:
For Forms, ensure that the Login page option is selected under How do users access their records? in the Share section. Portals will always have the login page enabled.
For a Form, select Share in the Builder toolbar. For a Portal, open Users & Login.
Select the login field.
Turn on Allow users to search from existing values in the table.
Your users will now see a magnifying glass icon above that login field that opens a list of possible values. This list can be filtered as described here.