CRJ-550 First Officer reported a miscommunication with the ground crew during pushback resulted in the aircraft rolling until the crew regained control of the aircraft.

Narrative

After the jet bridge pulled; ground crew gave brakes off signal; they were not on comms. Parking brake was released anticipating a tug connection. An extended period of time elapsed to secure passengers and make announcements before the door was closed due to late boarding. After a good amount of time; we finished our checklists and CA read for the brakes item; off". CA signaled for engine 2 start; then chocks out; then engine 1 start and it was at this moment the aircraft began to roll very slightly. Both myself and CA immediately responded and hit the brakes after the aircraft idled forward. The aircraft jolted from the stop CA set the parking brake. It was at this moment we called the FA. Cause: Lack of clear communication between flight crew and ground crew on procedures; erroneous signaling.Suggestion: Include in the charts information about power out gates or have ground crew plug in headsets so the procedure can be discussed and if any questions arise then they can be clearly answered."