Hi Marius,
thank you, much appreciated! However, in my case, I would like to keep the autocompletion to allow easy sharing within the same group, but not with other (third) parties. I agree that you provided a workaround for my initial question, but if I had a free wish I would like to have a separate option to limit shown contacts to those within the same user group 
Thank you for your help and best regards,
beeralo