// setup the callback
Client.Groups.OnGroupMembers += new GroupManager.GroupMembersCallback(Groups_OnGroupMembers);
// make the request
Client.Groups.RequestGroupMembers(Client.Self.ActiveGroup);

// handle replies
void Groups_OnGroupMembers(Dictionary<LLUUID, GroupMember> members)
{
    foreach (KeyValuePair<LLUUID, GroupMember> kvp in members)
        Console.WriteLine("MemberID {0} IsGroupOwner? {1}", kvp.Value.ID, kvp.Value.IsOwner);
}