It is not a bother at all :)

The first is about the group, this has rules of permanence? (For example, that it is required to participate every month in the event to stay within the group)

The only rule I have is to participate at least once every few months (every month would be unreasonable IMO as sometimes people just don't have time).

And the second is about a game, Slay the Spire. I started playing it on November 28th, this could be enter into this month's event, or not?

About games that have been started before the event. You only got 5 achievements before the event started (it started on the 30th of November this month as an exception) so it looks good to me to be added.

Also, related to the roguelike games, when would it be considered complete?

If you can complete a run in that game (the run doesn't just go on forever) completing one run is good enough to be counted as beaten.

If you have any more questions feel free to ask :)