A subpoena is not like a party invitation to which you can graciously reply that you have other plans. You are required to appear and tell the truth, unless the information you would provide might implicate you in a crime or qualifies under one of a few other legal exceptions (such as the lawyer-client or doctor-patient privilege).

But here's some good news: Because you are not an expert in workplace behavior, you have no obligation to comment on whether particular conduct did or didn't amount to harassment. All you're required to do is to answer questions about what you saw or heard that has a bearing on the dispute. For example, if you were asked whether you saw A touch B, you would reply yes or no and then, if you did see it, likely be asked more questions about when, where, and how often. But you would not have to answer questions about whether, in your opinion, A was sexually harassing B.

You have the right to be represented by a lawyer at a deposition, but before you go to the considerable expense of hiring one, check out Nolo's Deposition Handbook. After clearly understanding the rules, you may well conclude you don't need a lawyer.