I just got a level 58 pet and wanted to add him to my party, but the web page said it was to high a level for me to command. Why wont it let me do it?

There are pet levels and user levels.
Your current user level is level 29, which is based off of the highest level you've personally raised a pet to.
You can not use a level 58 pet until your user level is at least 58. (You can level the level 58 Griffi through missions or exp orbs to level 59 and it might boost your user level. I've heard about this method but I've never tried before. :O *uncertain* )