How do I respond?

So a close friend of mine just texted me, "Are you free this Sunday?" I want to say yes because I am. She's most likely organizing a hangout/reunion for our high school friend group (I'm in college now). However, there's this person I don't want to see so if that person's going, I don't want to go anymore.

How do I reply so that I know for sure who's going before saying I'm free? I just don't want my friend to know I don't like that person.