Sorry if it's a repost,but here it goes
1.Find a friendly guy from that guild and get him to talk to you.For example go to Durotar where everyone duels and duel a guy from that guild.Let him beat you and start asking questions about his class,skills,guild progress and so on...
After that thank him for telling all that crap and next few days spam him with "Hello!" and such crap.
2.On 3-5th day ( depends on how talkative that guy is) start asking questions about his guild and in the middle of the talk ask him how do they invite member alts to guild.(NB! Don't stop the talk after it,ask some more questions).Gather as much info about their guild as possible.In my case that guy told me that they just whisper their Guild Master or Officers from alts,telling who is his main and so on.
3.Whisper their Guild Master,saying that your the member of that guild and it's your alt( i told GM that i'm that guy's alt.I mean the one you are friends with,becouse you know much info about him and in case GM asks you something you can answer the question.)
And yes,that guy must be offline when you whisper GM.
That's how it worked for me and within one week a became full epic (had few heroic epix when i joined)
Good luck=)