When you run dungeons with players on your friends list, you will earn additional bonuses based on the different friends levels. As you can see from this list, it is good to have a bunch of friends who run dungeons with you to gain the extra experience points bonus to level up in Wartune.
Friendship Level 1: EXP +5%
Friendship Level 2: EXP +3%
Friendship Level 3: EXP +2%
Friendship Level 4: HP +700
Friendship Level 5: Charisma +70
Friendship Level 6: PDEF +280
Friendship Level 7: MDEF +280
Friendship Level 8: PATK/MATK +350
Friendship Level 9: PATK/MATK/PDEF/MDEF/HP/Charisma +100
As you can see, the stats boost at higher friendship level can make you pretty strong. If you can, try to stick with a couple of active close friends who play the game. The long term friendship growth will definitely pay off in both Wartune Arena fights and future group fights like the Wartune guild battle ground.