I did this a few months back with a team of 5 level 90s. Doing the initial Tanaan quests to get to building the garrisons took a little while - mostly getting setup with macros and all of the different classes working together. But once that was done, it was amazingly quick and easy.

With the routes listed, it's worth flying around and seeing all of the treasures so you know where to go when you have the potions on. And don't worry about missing some - you will easily hit 100 with the 3rd potion in Spires. Since you have to click on the treasures with all 2-5 characters, you will take a bit longer than those people doing it solo.

The objectives are the most important, so make sure you do all of those along with as many treasures as you can with the potions active.

Just hit up all of the listed bonus objectives at a team until you have 1 item or kill left, and then go the start point and take your potions and fly. In under 3 hours you'll go from level 91 with a garrison to level 100.

Good luck!