Nope, sorry i was wrong, made a mess when trying to apply this 
changes. everything is working fine.

I tried this with Kronolith 4.2.9 and Core 2.20.9, had to apply commit 
8d4b5affab357b732bffb737e00b0aff30b27751 (Add a 'beforeUpdate' 
callback), but it did not work.

I think the changes have to be made for the ResourceAutoCompleter too:
- I added the 'beforeUpdate' in the foreach in 
Kronolith_Ajax_Imple_ResourceAutoCompleter
- I added 'beforeUpdate' => 'function(a) { return 
KronolithCore.normalizeAttendee(a); }' to kronolith/index.php when 
creating Kronolith_Ajax_Imple_ResourceAutoCompleter

Kronolith 4.2.10 and Core 2.21.0

Changes have been made in Git (FRAMEWORK_5_2):

commit 46abfbb5690295c5fd161fe1a082854e0f75e359
Author: Michael J Rubinsky <mrubinsk@horde.org>
Date:   Wed Sep 2 14:02:17 2015 -0400

     Bug: 14093 - Deal with email groups in the attendee list.

  kronolith/index.php                               |    5 +++--
  kronolith/js/kronolith.js                         |   10 ++++++++++
  kronolith/lib/Ajax/Imple/ContactAutoCompleter.php |    2 +-
  3 files changed, 14 insertions(+), 3 deletions(-)

http://github.com/horde/horde/commit/46abfbb5690295c5fd161fe1a082854e0f75e359

Changes have been made in Git (master):

commit f5436e54700aac9762e9574b62c84220170eaef4
Author: Michael J Rubinsky <mrubinsk@horde.org>
Date:   Wed Sep 2 14:02:17 2015 -0400

     Bug: 14093 - Deal with email groups in the attendee list.

  kronolith/index.php                               |    5 +++--
  kronolith/js/kronolith.js                         |   10 ++++++++++
  kronolith/lib/Ajax/Imple/ContactAutoCompleter.php |    2 +-
  3 files changed, 14 insertions(+), 3 deletions(-)

http://github.com/horde/horde/commit/f5436e54700aac9762e9574b62c84220170eaef4

Confirmed. Probably just some validation tweaking needed for the group syntax.

When i try to add a contact list as attendees, i get this error: "No 
valid email address found".
But when saving the event, the contacts from the list are added correctly.