MyBB Community Forums

Long story short, I want my new members to post in the introduction forum first, have that post approved and then they get access to the rest of the site.
I know of making a new usergroup and promoting them after gaining 1 post count. But how to remove them from the old 0-post registered group then?
Also, is there any other way to achieve this?

Thanks in advance.

You need a plugin for this.
User Promotion System is what you have to use as marcus said below.

I don't understand why this can't work.

Registered group: deny access to all forums except one, then as you mentioned create new group that will have access to all forums and use promotion system to move user from registered to new group once they have reached X posts.

If not you need a simple plugin hooked to forumdisplay.php

Not tested!!!!!!!!!

$query = $db->simple_select("forums", "*");

while($forum = $db->fetch_array($query))
{
	if($mybb->user['postnum'] == 0 && $forum['fidX'])
{
error();
}