Contact Info (In-game, or other preference): Sthaphi or use the guild site
For or Against Multiple Guild Membership: Doesn't matter.
First thing you should know about that this guild main intention is to provide an environment where players can find easily party to do events/dungeons/WvW/etc.
The only requirement is that you have to represent the guild while you search for party in guild chat and while doing the actual event/dungeon/etc. Other than that you are free to do whatever you want, basically not represent the guild. If you don't respect this basic rule then you will be kicked because we are afterall Immortalls.
This being said if you want to join complete the application form on the website or PM in-game and you're in.