public static MembershipUserCollection FindUsersByRole(string[] roles)
    {
        MembershipUserCollection msc = new MembershipUserCollection();

        roles.Select(role => Roles.GetUsersInRole(role))
        .Aggregate((a, b) => a.Union(b).ToArray())
        .Distinct()
        .Select( user => Membership.GetUser(user))
        .ToList().ForEach( user => msc.Add(user));

        return msc;
    }

public static List<MembershipUser> FindUsersByRole(string[] roles)
    {
        var userList = roles.Select(role => Roles.GetUsersInRole(role))
                            .Aggregate((a, b) => a.Union(b).ToArray())
                            .Distinct()
                            .Select( user => Membership.GetUser(user))
                            .ToList();
        return userList;
    }

public static List<string> FindUsersByRole(string[] roles)
    {
        var userList = roles.Select(role => Roles.GetUsersInRole(role))
                            .Aggregate((a, b) => a.Union(b).ToArray())
                            .Distinct()
                            .ToList();
        return userList;
    }

List<string> roleset_to_find = new List<string>() {"RoleA","RoleB"};

            List<string> membersFound = new List<string>();

            foreach (string role in roleset_to_find)
            {
                membersFound.AddRange(Roles.GetUsersInRole(role));
            }

string[] roles = {"role1", "role2" };
        string[] tempusers = new string[]{};
        List<string> users = new List<string>();
        foreach (string role in roles)
        {
            string[] usersInRole = Roles.GetUsersInRole(role);
            users =  tempusers.Union(usersInRole).ToList();
            tempusers = users.ToArray();
        }
        foreach (string user in users) { Response.Write(user + "<br/>"); }