I would tread very carefully and very slowly..
Ask HR to provide the facts, ask questions about these facts.. and only agree to respond to questions if you really have to..
HR: We are here to ask you about the events of X evening..
You: OK..
HR: Mr B has alleged that he was bullied and ostracised by you
You: Can you explain what is meant by that?
HR: Can you describe your interaction with him
only if you have to..
You: Mr B asked me why I was not drinking, I did not wish to explain, I responded evasively and politely, he insisted with his questioning so I explained to him, clearly, that there could be many reasons why a person would not be drinking and might not wish to share those reasons with unknown colleagues in a professional setting. I felt uncomfortable being questioned in this manner but rather than showing my discomfort, I proceeded, as I have been trained to do, to clearly state my position. After this uncomfortable interaction, I moved to another group, and actually left the gathering shortly afterwards.
This questioning and insistence that I respond, left me feeling ill at ease and in a position where I could not easily decline to answer and move the conversation along to another more general topic. It felt intrusive to me.
Keep the answers to facts and how it made you feel.. avoid all use of 'squared up', stood my ground, saw red, told him it was inappropriate, patronising tone, forthright stance, unblinking eye contact, standing at full height, no smiles etc. also avoid things that you did not do.. shout, swear, overtly aggressive.. (as in for toddlers - when they hear don't touch the knife - they hear - touch the knife.. or if someone asks you if you are scared - your mind thinks there is something to be scared of so you using the words 'shout, swear, aggressive', will as others on here have done, bring a question into the interviewers mind that maybe you had been shouting, swearing and behaving aggressively.
No matter how many times you are asked - repeat - I was calm, as I am being now, I explained clearly, as I am doing now, I left because it had become uncomfortable - again - the use of comfort - links comfort with you...
Good luck - practice this with someone before your meeting and keep a calm tune in your head....
If you are asked about potential outcomes - you could mention that you are concerned that this will appear to have been something that you have done wrong and you wish that it be made very clear that you are being interviewed as a witness and not as a party to the complaint. If they suggest that the colleague has complained .. remind them you had nothing to do with the circumstances around the complaint, in fact, you have done absolutely noting wrong. You felt uncomfortable and left, you are irrelevant to any subsequent event when you were not present nor any subsequent behaviours of anyone else at that gathering. Avoid mentioning his name, this person, this man.. distance yourself entirely from the original event - you were not there..