I believe the SEAL team knew they were going after Bin Laden from the time they were in the U.S., but not the way the movie portrays it. Plus, I don't think a senior SEAL planner would keep it a secret from the team. The team already had 10 deployments under their belt. It would be nice to have something special to motivate them.
"...another source referencing a kill order stated, 'Officials described the reaction of the special operators when they were told a number of weeks ago that they had been chosen to train for the mission. They were told, 'We think we found Osama bin Laden, and your job is to kill him,' an official recalled. The SEALs started to cheer."
"The CIA briefed Vice Admiral William McRaven, the commander of the Joint Special Operations Command, about the compound in January 2011. McRaven said a commando raid would be fairly straightforward but he was concerned about the Pakistani response. He assigned a captain from the U.S. Naval Special Warfare Development Group to work with a CIA team at their campus in Langley, Virginia."