One thing to remember about posting on these groups -- pay close attention to the rules of each group. Some only allow you to post in specific forum threads, while others only want you to post bulletins. Follow the rules or you risk being kicked out of the group.
Following are links to the most popular Add Me Groups on MySpace:
Add Me Group
Members: 87323
The 'In' Crowd
Members: 36209
MySpace Whores
Members: 16494
Add To Friends Network
Members: 11431
Whorespace
Members: 9635
The MySpace Whores
Members: 5859
MySpace Friends
Members: 3035
Add Me Cuz I Rock
Members: 2904
*wAnNa NeW fRiEnD*
Members: 2115
Anyone and Everyone Add Me Group
Members: 1876
ADDiction
Members: 1648
MySpace Friends
Members: 1081
Want More MySpace Friends
Members: 1146
1000+ Friends!
Members: 848
Get your Whore Me Codes:
Picgames.com
MySpaceSupport.com
FreeCodeSource.com
For more info on adding MySpace Friends read:
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Coming soon... MySpace ADDiction - Friend Trains