Isn't that exactly the kind of message the browser popup already displays when it asks to share your location? What would the interstitial message add to this? Plus, not all browsers support location services.
Yes, but if a user click anywhere on the page the browser message can disappear.
An option for the admin to add or change the message could explain what the map is about.
The browser message for location sharing does not disappear when you click anywhere on the page other than the Accept or Deny button, in Chrome at least. Other browsers may have other standards, but there's no good reason to break with browser standards as that's what users are used to from other sites.
The user will have to go out of their way to permanently disallow the member map to use their location, so an explanatory message seems a bit superfluous. Forcing users to go through extra steps in order to see your content is usually a very bad idea