Use OG Contact Form to Broadcast to All Members

Currently, no that is not possible. In the release I'm working on yes it could be made to do that, as I'm adding the ability to send to members of role with "receive contact form..." permissions.

However I should point out two things. Organic Groups already does this with its Email tab, which does exactly what you are requesting (though only admins can send messages).

The other is that I would proceed with extreme caution in opening up what is essentially a spam engine to untrusted users. If you have a couple groups that have ~100 recipients, and a contact form bot finds your site, we're talking hundreds of emails an hour that they can send.

Anyway, I'm postponing this request, as its not really a duplicate of an existing feature request, and I am actively planning to make this option possible. Follow the upgrade thread for more information.

I wouldn't do this if I were you. It is a direct duplication of the broadcast function in og and there are issues as you describe. Og contact form does a simple task very well. Keep it simple! KISS!

The 6.x version of OG Contact has the ability to send to any group member that has the right permissions. That should accomplish what you want if you configure all your users to have that permission. This may not get back ported to the 5.x version.