In case you are wondering what this group is ment for:
Any admin/moderator of the group can create events or in other words 'organise' a multiplayer session. They can set whatever rules they wish, have as many players as they wish and use whichever DLC or mods they want to use for it.

If a you wish to join a multiplayer session, then you can simply comment on the event that you would like to join and speak with the host about wether or not that is possible. Note that the session should be set to a certain day and time of the week on which most players should always be able to join in.

Now why would you join in a game in this way instead of doing it the normal way, just by browsing the games normally in the multiplayer section of the game or by playing with your friends?
For the obvious reasons of continuity and variety. By organising the sessions this way, it is ensured that the session won't just be a one time thing but will be continued over the time of a few weeks or even months. Furthermore you will be able to play with more people than usual, because of the organisation of sessions at specific times.

So in short, these sessions will hopefully provide continuing, diverse multiplayer games of EU IV for a lot of people and I wish you all the best of luck in future games!