It's not time to spend money on offsets for others until you've cut every expenditure you can. Carbon offsets can come dangerously close to greenwashing very easily: we recommend you google carbon indulgences and look at cheat neutral to see what your serious environmental attendees will have already read. So carbon offsets are great as a sweep-up at the end, but be careful how much you claim if your event isn't green and you just indulge in offsets.
When I first heard of carbon offsets, I thought they were wonderful. Barbara offset the car she couldn't avoid driving, and used the sticker to start conversations with neighbors. It was the beginning of her climate activism, not the end. Today, carbon offsets are often being renamed as Carbon Indulgences: not only can you offset your fuel efficient car, now you can offset your private jet. In many hands, it has gone from a first step towards real change, into an excuse and last step. I don't see Carbon Offsets as worthless, but we do think they are over-hyped and not nearly as effective as proponents claim.
There are no good numbers on carbon offsetting, and never will be. It's not a math problem: the impact of your conference will vary by something like a factor of ten depending on the choices you make about what to include. Are you counting the lighting switched on for your conference, or the share of carbon pollution from making the planes to carry your attendees?
There's very little reason to give your money to a carbon offset company. We are fans of do-it-yourself offsetting. Donate CFL lightbulbs in the neighborhood of your visit. Find a charity and donate the amount you're ready to donate, without a middle-man taking a cut. Send us ideas. Researching carbon offset companies is probably harder than researching nonprofits, anyway.