There are several ways to find out about an event.
- You can run into it yourself.
- A friend can tell (or /tell) you about it
- Someone can mention it in the guild channel of his guild
Not difficult to see that that 3d way can get half a guild in an event easily, especially if large part of the guild is on-line, or there are other communication means (guild forum) and it is a longer running event.
I still have to run into an event, or be told about it by a friend, as I am not in a guild and I am convinced GMs who I know will not tell me or others in advance.