Although many works have been dedicated to standard protocols, very few address the more challenging class of group protocols. We investigated group protocol analysis in a synchronous model, that allows the specification of unbounded sets of agents with related behavior. Also, when used in an asychronous way, this generalizes standard protocol models with bounded number of agents by permitting unbounded lists inside messages (including unbounded number of variables, nonces, etc..). This approach also applies to analyzing Web services manipulating sequences of items. In this model we propose a decision procedure for the sub-class of well-tagged protocols with autonomous keys.