Yes, indeed. Ambassadors and higher can grab the flag.Strange, on a life server I am an ambassador and in charge of the FSA.
I can't unlock the stage prize so I get promoted every FSA to mage.
(In fact, for my own personal account, which shall remain nameless, we had an ambassador grab a flag in the last round of Adventures we had, here in Beta )
If you happen to have any issues with this, you can let your live Support Team know with the next round, or send us a Bug Report when Beta has another Fellowship Adventure.