Guilds are all Cross Server since the Mega-Server change a year or two ago but Server does matter to who you World vs World with. We have been playing Yak's Bend but it is full and no new players can join that Server.
You may join the Guild on any Server but if you would like to WvW with us I have started a second account on Crystal Desert and have been directing new DbD Members to go there.
I am considering transferring my main account there as well but I wanted to speak to a few that are still playing on Yak's Bend first and of course it is depending on interest of the Guild in WvW.
We have had a Guild on Yaks Bend for quite a while and we fought for them a very long time bring server rank up to top tier.