Hi Vera,
I looked at adding this as a feature to the choice here:
http://moodle.org/mod/forum/discuss.php?d=24656
be careful with this patch though as I haven't tested it, and the choice module has gone through quite a few changes since!
it was decided that the Choice module was the wrong place for this, that it should be included in the groups management.
there's a patch somewhere in this fourm that adds it to the group end -
not sure what version, or whether it works still - make sure you test
these patches in a non-production environment as they may break
something.
http://moodle.org/mod/forum/discuss.php?d=6247
Also - if you were able to arrange funding for this development, you
could contact moodle.com or one of the Moodle Partners and they may be
able to develop something for you that works!
good luck!
Dan