The promotions system is pretty flexible and comes in handy, for this tutorials I will give you a situation and how to make a promotion to handle it.
You site has been hit with a rash of people who sign up and spam with PM's but dont post or make only 5 posts or so....
You can cut all that out with a simple promotion
Go to your "usergroup manager" under usergroup on the left menu
Add a usergroup called "Members" and set access levels to what your registered members have.
EDIT the registered usergroup and restrict PM access totally.
Go to usergroup menu on left and click "Promotions" then click "Add new promotion"
Once there in the top dropdown pick "registered members"
next set posts to whatever you would like, say 20 posts.
For Promotion Strategy dropdown pick "posts"
For the type pick primary
For Move User to Usergroup pick the "Members" group you made that DOES have PM access.
SAVE
Now new registrants will have to make 20 posts to PM people, this is because they have no PM access as "registered members" and after 20 posts are automatically promoted to the "Members" usergroup that DOES have access.
There are many uses for this system, be creative...