Stuck on the mission "It Takes 2-4".

I'm stuck on the mission where you need to party up with a group and complete three different co-op challenges. Even when I invite people to my party, they accept, and I head to a challenge, I'm unable to select "Start Co-op". Underneath "Start Co-op" says "Party leader only", but I am party leader. Does anyone know a workaround, or can someone invite me to their party and help me finish these challenges? Thanks.