Thinking sideways - rather than using guest ambassador, have you considered using "Sponsored Guest" instead?
With this approach guest users use the splash page and sign up. They have to give the email address of someone who works at the company (their sponsor). That person then gets an email with a link to click on which approves that guest. No login to the Meraki portal required.
If you want, you could put on a note on the splash page asking users to always put in a specific email address, or if you try a little harder and customise the page, you could probably just hard code an email address to.
Voila, no guest ambassador account required.
Note: You have to open a support ticket and request Meraki support to enable this feature.