More Fun With Friends

Did a Mythic dungeon (Slags) with 3 guild members last night, and it counted toward this achievement (according to my armory).

I submitted a screenshot for documentation.

If you and a guild mate are a good combination (say a DPS and healer) and it's just the two of you, you can have anyone join as your third person and enter a scenario.
When it starts, have your extra non-guild person leave the group. You get guild credit since it's only two completing the scenario are in the same guild.
Even if you offer the person 50 gold to help you for 30 seconds, you still get 250 gold for completing as a guild the first 15 runs. Profit.