For Marcus/Juliet: for Marcus you need to ignore his advances, not talk to him, play the "hard to get" girl. For Juliet instead you need to talk about Lora when you're captured, ask her about the sword, and so on (be social).
You can find out if they're with you or not in the final scene when they gather the crew and announce what they want to do.
If they're with you they'll say "After thinking carefully, we've decided to keep Heileen and her friends onboard with us."
If they're NOT with you they'll say "After thinking carefully, we've decided it's best to leave Heileen's friends on the next island."