There always were guild ambassadors at Silan as far as I remember. I don't see it as a problem. It's not like you're stuck with a guild forever once you join one. Really, this server isn't big enough to be bothered with guild politics.
I'm sure there's a technical possibility to prevent Silan players from joining a guild. But I really don't see the point.
I'm sure there's a technical possibility to prevent Silan players from joining a guild. But I really don't see the point.