I truly believe grouping is at the heart of what makes MMO's successful. But, you cannot just force player interaction or believe that your players will see the value in grouping with "Class XYZ" just because. In EQ, there were some class abilities/spells that were so desirable, that they almost became a must or a foundation to start any group on. Why not apply this thinking to all classes and make it so everyone has an incentive to group with everyone else?
Every class should have signature Group Aura abilities that makes them desirable a group. These auras only become available once you are grouped. The toggle for which one you select has a 5 minute cooldown, but the ultimate goal is to make is fun(ner) to group with any other class. Multiple of the same aura's should stack, with diminishing returns. Every 10 levels, there should be a class progression quest to upgrade your aura to the next tier.
I will use EQ as an example. Each group member can select one of the following abilities:
Enchanter
-Group mana regen increase
-Chance on being hit for the mob to be put into a slumber for 2 ticks
Wizard
-Group spell damage increase
-Ice shield aura that has chance upon being hit to root the attacker for 1-3 ticks and slows their attack speed
Magician
-Group damage shield and increases resistance to Magic
-Chance to crit direct-damage spells
Necromancer
-Group damage over time damage buff and increases resistance to Disease + Poison
-Melee proc, raises an undead companion that strikes your target for the duration of 1 tick until till it's bones shatter and return to the soil
Bard
-Spell reflection, chance to have a negative effect spell reflected back to the caster
-Group mana spell cost reduction
Rogue
-Group attack rating buff, allowing group members to hit more frequently for higher amounts
-Ability/spell cool-down reduction
Ranger
-Group experience buff
-Group melee evasion/avoidance aura
Monk
-Group double/triple melee attack buffs
-Chance to dual cast spells
Beastlord
-Group companion attack speed, dmg proc, and regeneration buffs
-Group health and mana regen
Beserker
-Group agro reduction
-Group agro enhancement
Shadowknight
-Group lifesteal melee proc
-Group chance to feign death when fatal damage received
Paladin
-Group rune shield that replenishes every tick and blocks a lower amount of damage
-Group rune shield that will block any amount of damage for 5 occurrences on a 2 minute recycle
Warrior
-Group AC buff
-Chance to channel through hits/bashes to cast spells/abilities
Druid
-Group regeneration and increases resistance to Fire + Cold
-Group melee proc, cast a snare that lasts 5 ticks
Shaman
-Group melee haste that stacks with haste buffs
-Health to mana conversion aura
Cleric
-Group spell haste
-Group melee proc, direct damage with mini-stun