Ruling on being friendly towards someone who is drinking alcohol, and pouring his drink for him

We have no choice but to
attend parties held in the work place (a school laboratory), where people
drink beer and other alcoholic beverages. But praise be to Allaah, we do
not go anywhere near those drinks. However, they have a custom whereby a
person has to be nice to the person next to him and pour drink into his
cup (beer or whatever he is drinking), and that person returns the favour.
So they pour juice into my cup for me. What is the ruling on my pouring
beer into the other person’s cup? Will i be considered as one who offers
(alcohol)?