Rebel usually hang out at The Grotto in Sims University.
Work on your Sims Painting and Street Art skills, use Megaphone, eat herbs and drink organic brew from barista bars; will eventually gain your Rebel social group influence.
Using "Spice it Up" option during Bonfire will increase your Rebel influence. These herbs have special/different effects on your Sims!
Line Crosser (level 2)) will receive the Street Art Kit and a Megaphone for free. Your Sim can also steal exam answers (either ask the answer from the other Sims or look for exam answer from the computer) and sell them.
Free Thinker (Level 4)) can use the Megaphone to organize a Hunger Strike protest, and use the podium to ask "What is Wrong with the World?". It also become easier to get away with "Cheating on Exams".
Agitator (Level 6) can use "Show off New Gizmo" interaction, organize "Rowdy Protest", and unlock new "Rebel" smartphone skin.
Maverick (level 8) will get an additional Social Group Trait. Reaching level 8 with other groups will only gives you an option to replace the existing bonus trait.
Trendsetter (max level within Rebels) will unlock the Art Appraiser Career (split into two branches at level 6).