How to filter activities from the list of users through webservice

Hi,

Using web service with C#, I try to get activities where the user is from the list of users.
I mean if one activity has many users attached with. I want to be able to filter my query on that users list (and not on the Activity OwnerId field) and get activities result based on this condition.

How can I do that? Thank for your help.
Regards.

This is my code :

string strResponse = "";
// Create an instance of Activity
Activity myAct = new Activity();
myAct.Url = mSession.server + "/Services/Integration;jsessionid=" + mSession.sessionId;
// Values needed back from the server
queryType myActID = new queryType();