Groups make it easier to organize and find buddies.
To create a group click on Contact on the menu bar then Add Group...
Enter the name you want for your group and click on Add.
The other option to Move someone is:
Right mouse button click on the person you wish to move. (or Contact on the menu bar Get Info)
Click on the fourth tab.
Click on the + button in the lower left corner and select the group you want to move the person to:
If you had already added this buddy to a group you may want to remove them from the old group.
Click on the old group and then the "-" minus sign. Close the window by clicking the "red" close button.