The paper proposes control schemes for a multi-quadrotor system flying in formation. The underactuated quadrotor model is highly nonlinear. The control objective is two-fold: To maintain the formation structure in flight along a geometric path and to follow a timing law that dominates the rate of advancement of the group and the arrival times to an assigned site. The paper suggests control strategies for a string-like formation (where all vehicles involved are linked by a virtual thread) and for a rigid formation. The concepts of flat system and virtual formation play essential roles in the current approach. We assume that whenever needed free flow of accurate information between the group members is available. The paper is concentrated on the low level relevant control issues. The leading vehicle governs the overall group motion. However, the control scheme allows formation splitting and merging (assuming the relevant data are available by communication/sensing means) during the group motion. Simulation results conclude the study.