Well... I found a usefull solution myself.

Did an event and created a delay of five seconds, which gives Umbraco plenty of time to update the membergroups on Member.

Here is the solution:

public delegate void MemberSavedEventHandler(object sender, IMember member);

public event MemberSavedEventHandler MemberSaved;

And then let the MemberService_Saved invoke a Task named "UpdateMemberAfterSaveDelayed(member)", which then waits the five seconds before running.

    /// <summary>
    /// After a member is saved.
    /// Debtor Data in e-conomic is updated.
    /// </summary>
    /// <param name="sender"></param>
    /// <param name="e"></param>
    private void MemberService_Saved(IMemberService sender, SaveEventArgs<IMember> e)
    {
        foreach (var member in e.SavedEntities)
        {

            System.Threading.Tasks.Task.Run(() => UpdateMemberAfterSaveDelayed(member))
            .ContinueWith(t =>
            {
                var ex = t.Exception.InnerException;

            }, TaskContinuationOptions.OnlyOnFaulted);

        }
    }

    /// <summary>
    /// Delegate that triggers RegisterEvent_MemberSaved after 5 secs.
    /// </summary>
    /// <param name="member"></param>
    private void UpdateMemberAfterSaveDelayed(IMember member)
    {

        System.Threading.Thread.Sleep(5000);
        if (MemberSaved != null)
        {
            MemberSaved(this, member);
        }

    }

    /// <summary>
    /// Be aware! This method is running in a thread. Not all classes can be instanced.
    /// </summary>
    /// <param name="sender"></param>
    /// <param name="member"></param>
    private void RegisterEvents_MemberSaved(object sender, IMember member)
    {

        if (member != null && member.Id > 0)
        {

            if (member.IsDirty()) {

            ..... doing all kinds of stuff....

            }
        }
    }

May you also can use this...