Cryptographic protocols are crucial for securing electronic transactions. The confidence in these protocols can be increased by the formal analysis of their security properties. Although many works have been dedicated to standard protocols like Needham-Schroeder very few address the more challenging class of group protocols. We present a synchronous model for group protocols, that generalizes standard protocol models by permitting unbounded lists inside messages. In this extended model we propose a correct and complete set of inference rules for checking security properties in presence of an active intruder for the class of Well-Tagged protocols. We prove that the application of these rules on a constraint system terminates and that the normal form obtained can be checked for satisfiability. Therefore, we present here a decision procedure for this class.