ECL is the sum of Hit Dice, level adjustment, and class levels.
If I'm going to play an ogre mage, I'd need to select a class, probably wizard, and start at 1st level in that class. So a 1st-level ogre mage wizard would be HD 5 + LA 7 + Wiz1, or ECL 13. He could join a 13th-level party just...