Not for a sitewide notice, but you can pick from two plugins that give you the ability to sent an email to all users, these are Mass Messaging in BuddyPress and Mass Messaging for BuddyPress by Alkaweb.
thank you for the fast reply. Now I use “Mass Messaging in BuddyPress” and it worked fine.
Unfortunately, most Users have the problem that the emails end up in their spam folder.
This might be because the emails are coming out with a WordPress email address, you could try using something like CB Change Mail Sender to change the address to one specific to your site maybe?
I don’t want to use too many Plugins you know about an option to change that directly in WordPress or the Database?
Try asking on the WP Forums maybe?