And logging off and back in and your console? Maybe you have to run around a bit more, or enter/leave the dungeon more times until it triggers. But usually the “explore” step means to kill the main boss. It’s apparently bugged, I can only advice you to try it more times, each time after switching to another journey and then back.
And yes, you can finish the journey with a clanmate. I believe you two killing the boss together is enough to finish this step, only both of you have to at least hit the boss before he dies.