Potential new player

[Rhonya]

If we do communicate with Alliance, usually that's done through Real-id conversations indeed where you add the people involved into, and talk and emote through that chat, but it's a very messy procedure if you have a lot of people involved.
Ingame otherwise there are no ways to really communicate with them, no.. apart from the standard emotes that both sides understand from one another.

[Alfa Kilo]

Are guild members allowed to have alts in the opposing faction? If so, how does that work in terms of game mechanics (does the guild name still show under the character, is the alt allowed to join an alliance guild on the same server,...)?

[Srelok]

Guild membership is personal to one character. So all characters can be in different guilds.

[Alfa Kilo]

Ah, amazing. For some reason I thought it's account-wide (was it so in the past, maybe?).

[Revax]

It was never that way no

[Srelok]

that was in Guild wars (1 and 2)

[Alfa Kilo]

That's probably the culprit then. I used to play both.