have you got an office manual with the relevant procedure? you could ask HR for a copy if not.
if you are the object of the investigation, then you should be able to ask for an independent representative such as a trade union rep or another colleague to sit in and make notes.
If you are just a witness, then i doubt you can have another person in there to assist you.
In both events, you should then be given a copy of your statement to approve and sign afterwards to make sure that the transcript of what was said is correct.