Set Default Sharepoint group using webservice

Hi,

I'm using a workflow to create a site based on a template and some info, in this workflow I create 3 groups ( Owners, Members, Visitors) with their permissions.

I only have one problem. These group are not set as default group like we would do using this page : /_layouts/permsetup.aspx or by using associatedmembergroup from the spweb object.

Is there a way of doing that with web service ?

Regars,

Loic M.