Make Member a User

wpm is built on top of wp user…rtfm;-)

WP-Members uses the WP database schema, so all users are WP users.

Any custom fields are stored as user meta, just like any other custom field. This allows for maximum compatibility with other applications since you can make your custom fields use any necessary meta key name so as to match up with whatever plugin you are integrating with.

If this is true then I have a worse problem than I thought. I appear to have two users who are in the WPMembership list but not the WPUsers.
I have tried searching for them in WPUsers by username, name, email address etc but they are not there.
Any suggestions how I might resolve this?

Sounds like you have a plugin other than WP-Members.

https://wpusersplugin.com/

Oops, you are right! Wrong plugin!