that isn’t really a “kind of OOC warning”
A “kind of OOC warning” would be to whisper the event hoster / Rogmasha to inform them of you and your guild’s plans. A bunch of people showing up unannounced, attacking during an event they’re doing doesn’t sound like an out-of-character warning, but I know you might get the two mixed up / confused. 
doesn’t seem v. hard to me personally to toss someone a /w before you bullrush into a bunch of people’s events and make a lot of people unhappy because it felt forced and awkward to be the recieving end of.