My dog and I walk with a group of people & dogs, all leashed. One of the dogs is extremely reactive and lunges at my dog. How should I handle that?

Who organized the group? Honestly, it sounds like this other dog needs to work on his issues away from the group setting before being expected to handle himself in this scenario.

I would ask this person to work on their dog away from this group, they are only making their dog's issues worse by putting him in this situation that he clearly isn't able to handle right now. If they wouldn't agree to this then I wouldn't walk my dog in that group anymore.

My job is to advocate for my dog as much as possible and if you know this other dog is going to be there and the owner won't do what needs to be done I'd find another group of balanced dogs to walk with.