Circle of Elders

• WOTC Official Reminder Textofficial rules
  Vigilance — Attacking doesn't cause this creature to tap.

• WWizards of the Coast2015-02-25
  Official Ruling
  If you control a creature with power less than 0, use its actual power when calculating the total power of creatures you control. For example, if you control three creatures with powers 4, 5, and -2, the total power of creatures you control is 7.
• WWizards of the Coast2015-02-25
  Official Ruling
  Some formidable abilities are activated abilities that require creatures you control to have total power 8 or greater. Once you activate these abilities, it doesn’t matter what happens to the total power of creatures you control.
• WWizards of the Coast2015-02-25
  Official Ruling
  Other formidable abilities are triggered abilities with an “intervening ‘if’” clause. Such abilities check the total power of creatures you control twice: once at the appropriate time to see if the ability will trigger, and again as the ability tries to resolve. If, at that time, the total power of creatures you control is no longer 8 or greater, the ability will have no effect.

Rulings via Scryfall