Hi, sry for the late reply. Talk to Mike, exhaust all dialogue, visit the brothel a few times and exhaust all of Stella's dialogue. She should join your fighter-team soon after.
You need to have Stella join the gym (by defeating the first boss at the Streets of Rage), then do Stella's first training session together with Gina. After that Greg should have a new dialogue-option about sharing those shakes.
