Hey there, unfortunately outside of Lightfall/The Final Shape campaign missions, and Raids/Dungeons, mission checkpoints are there only for respawn purposes during the current session. They do not allow you to quit and load back into them. Is this the 'Mesmerize' mission? You can take a fireteam into it, but, (another 'unfortunately') you can only take people in who have never completed the mission before. You would need to find people who are at the same step, players who have completed the mission already can't enter the mission again.