[How To?] New users notification for post

[WallBB]

I don't know of any such direct plugin but you can do this using two ways :-
1. Template Conditional plugin
2. Use any guest plugin and modify the plugin to show guest message to users with less 20 posts (or any numbers). Use group promotion feature in MyBB to promote the usergroup after 20 posts and hence they will not see the message.