LocalSettings.php

1. Disable for everyone.

$wgGroupPermissions['*']['edit'] = false;

1. Disable for users, too: by default 'user' is allowed to edit, even if '*' is not.

$wgGroupPermissions['user']['edit'] = false;

1. Make it so users with confirmed e-mail addresses are in the group.

$wgAutopromote['emailconfirmed'] = APCOND_EMAILCONFIRMED;

1. Hide group from user list.

$wgImplicitGroups[] = 'emailconfirmed';

1. Finally, set it to true for the desired group.

$wgGroupPermissions['emailconfirmed']['edit'] = true;