Instructions
Walk up to a friend in the "Party" section and press the "X" button on your PS2 or "A" button on the Xbox 360 to talk.
Have your friend select the "Invite Bot" option.
Allow your friend to select the "Switch Party Leader" and then once he does this, you must press the "Yes" option quickly five times. If done properly, the "Form Party" option will show up on the screen.
Select any one of your available bots from the list and confirm the selection by selecting "Yes." Instead of your bot, however, you will receive your friend's level 80 bot as the party leader. The bot is yours to use as long as your friend does not leave the party or switch leaders.