The fact that the circle is filled in means selected, and therefore share. I have no question about that. Name is selected here.

Whether the X has additional meaning, like "this is a thing you can't uncheck," I don't know.

I agree it should be redesigned.

From my iOS experience I would expect the 'X' to delete all of the text in the field, but it's very confusing considering the toggles below it.

I agree, my interpretation of that user interface would be that X clears the field and lets you type something else.

I'm am iOS user and I have no idea. Could it mean that it's mandatory to share? As in, it's selected and you can't unselect it?

These types of ambiguous toggles drive me nuts.

It's just to clear the text.

I doubt that's it. First, it's the same size and shape and position as the selectors below. Second, the usual "clear text" X only shows up when the input box is in focus. That doesn't look to be the case in the picture.

I think it is to clear the text, thus not giving your full name to the app. I assume when the email is selected it’s not filled with an x but a check mark.