Icon is clickable and will use Class Buff for Mage (Arcane Intellect), Warrior (Battle Shout) and Priest (Power Word: Fortitude) .
The Icon will be visible whenever any players in your Group/Raid don't have your class buff and are within 100 yds of the player and are not dead/ghost/DC.
Hovering over "Buff Missing" will show the names of the players missing your class buff (includes everyone in the group not only those in range, Disable to reduce CPU usage).
Reload your UI after importing the weakaura.
Alternative Weakauras: Only icon for combat https://wago.io/HkpzjiGB7 and Clickable while not in combat https://wago.io/rJgI_cH4m
Update version 18: Fixed errors
Update version 16: Simplified conditions
Update version 12: Update range check
Update version 10: Decrease CPU load, more efficient and simplified version
Update version 9: Icon no longer shows when dead, Disabled in combat to increase performance in fights.
Update version 8: Changed range requirements to class buff spell ranges, usually 100 yds (weakaura uses "IsSpellinRange" which only tracks the x and z axis, so its possible the icon shows up if someone is above/below the player yet not in range, If this problem bothers you please use version 7 which tracks everyone in 40 yds radius regardless of height).
Update version 7: Added level restrictions, the icon will no longer show if player or party member is below required level for class buffs.
Update version 6: Slightly modified code used from Zauberjockel (https://wago.io/By8pPJk47).
Update Version 5: more stable and efficient version.