I just happened to provide a short example of setting up a custom role capability (exaplanation and code). In your case, however, you want to add the capability to particular users – not roles.
The following code may be a starting point for what you want to do:
>>> Setting it up
// The IDs of the `privileged users`
$users = array( ... );
// Required arguments for the user query
$args = array(
'include' => $users,
'fields' => 'all_with_meta',
);
// Add the capability to privileged users
foreach ( get_users( $args ) as $user )
$user->add_cap( 'group_message' );
>>> Using it
// Check for the capability
if ( current_user_can( 'group_message') ) {
// group message stuff
}
>>> Cleaning it up
// Remove the capability
foreach ( get_users( 'fields' => 'all_with_meta' ) as $user )
$user->remove_cap( 'group_message' );
All you need to do now, is think about getting/providing the user IDs. You could, for instance, list all users on your plugin menu page (if you have one), then select the ones you’d like to be capable of group messaging, and finally add the capability to those users.