The great thing about using a pet is that not only is the fight always kept at a distance, but you are a mini-healer and able to heal your pet.
Necro's also have a Life Tap ability to keep themselves and the pet alive.
Conjurers are typically better against groups, while Necro's are typically better agsint single targets.
Conjurers have 3 types of pets:
Earth/tank: high HP, low DPS, great Taunts
Air/Scout: Medium DPS and HP, fast attacks
Fire/Mage: High DPS, low armor, AE attacks
Typical grouping with multiple pets is to make it up similiar to a standard group. One will have a tank pet, the other will have a Scout or mage pet. Then the 2 casters will back it up with additional DD and DoT. Necro's are poison and diesease damage, Conjurers are elemental and magic damage.
Pets also allow you to make "surgical pulls" from long distances. Target a mob, pet-attack, and watch it sprint halfway across the zone to get it. Then pet-back-off, and it will run back to you, dragging the mob with it.
Be careful with this, as some mobs are social agro.
The closest I ever got to a full pet-group was 4 pets with an Enchanter. We did not have a healer and didn't need to. We rolled through any mob we came across, and if the mob broke for the casters, the Enchanter would mez it until we could resummon the pets. It was beautiful.