@Gaia Grant - It is my understanding that the prompt you are showing is a feature that is still in beta and therefore is not available on every paid/Education plan team and could also be temporarily removed while updates are made.

There is a workaround that has existed for a while, which is to start the board in Anyone with the link "can comment" mode, have your guests leave one comment at which point they can every their name, and then you'd flip the board into Anyone with the link "can edit". Definitely not as slick as the beta feature, but it'll work - details here:

https://community.miro.com/best-practices-discussions-48/trick-show-the-real-name-of-an-anonymous-guest-editor-2515

I think this workaround “Anyone with the link "can comment" mode,”  has been removed. Pretty tough trying to write instructions to new users on a board when these things keep changing. one week this is a feature, then its gone with no explanation. 

@Gaia Grant - The workaround still works, but I’ll admit that is it not very user friendly - it’s more of a hack

Hopefully Miro officially launches this guest-naming feature soon.