To join (BF3) SEAL TEAM 6!!
Go to http://battlelog.battlefield.com. and sign in.
Under the Platoons link click ( Find or Create Platoon)
Next go to the bottom right of the page and search for
(SEAL TEAM 6!!). 
After you get the list, ours has a skull with the number 6 next to it. Click the link.
Then at the top right you will see a button  (Apply to join).
After you have applied look on the right top corner where your name is under the miniature avatar click the drop down and choose (Edit profile).
On your Edit page you will see something like (Soldier settings Edit soldier image and clan tag:)

  Yourgamertag

In the box type in ST6

 Then scroll to the bottom of the page and click save. 

That's it. You are now part of the Platoon. Your rank and Squad will be assigned at a later time.

Thank you for your time.

Merci.