I need to represent just that.
My english is very bad, so I show you one E-R diagram:

---------- ----------- ---------
| Name |------------->| Name | | |
---------- | Event |<--------|Event |
| Posts | ---------
| |
-----------

One user could be register in one or more Event, and in one Event could be registered one or more users. To here I can do, but what happen if I need to remember the number of posts that one user made in one event.

I hope that you understand me, and thanks for read,

Check the ternary association.

You'll have
User
===
one-to-many UserEventPost

Event
====
one-to-many UserEventPost

UserEventPost
=========
many-to-one User
many-to-one Event
many-to-one Post

If Post can handle many-to-one to User and event, then replace UserEventPost by Post