When you group, the pet takes a minimal to zero experience share from your party. This is different from soloing - when you solo, your pet does take a percent of the experience that you would ordinarily have gotten. Pets will take 50% of the experience from a kill if they have done over 50% of the damage to the monster.
On average, a pet will take about a 0.5 to 1.0 (Out of 100) percent party experience share. From personal summoning experience, a pet is always worth having in a group if you are in an environment that is pet friendly.
When the /pet guard here command is used on a pet it will always turn to face the nearest monster. (Very helpful to see if a mob is coming your way.)
