Two trajectory tracking control strategies based on the leader-followers scheme for a group of differential-drive mobile robots are presented where the output to control is the mid-point of the wheels axis of each robot. A third control law is presented to avoid the singularity when the longitudinal velocity of any robot tends to zero. A previously reported observer, based on the Immersion and Invariance technique, is used to estimate the attitude of each robot. It is shown that the combination of the two trajectory control strategies with the vehicle attitude observer yields an asymptotically stable closed-loop system, while the combination of the third control law with the attitude observer produces exponential convergence of the longitudinal velocity errors, asymptotic convergence of the observation errors, and bounded tracking errors. Finally, real-time experiments exhibit the adequate performance of the control strategies proposed despite tracking trajectories that present unfavourable operating conditions for the robots.