Talk to your boss's / the people making the decision.
Explain that the decision has caused a divide in the team.
Ask if other people will be able to attend once the budget is bigger. People don't like being left behind in their careers, so it's important to make it look like they aren't being forgotten.
But that's something your boss's will have to tackle, the choice about who goes isn't yours to make, so the fault is not yours either.
There are, however, a few things you personally can do to make sure you aren't causing this negativity to stick around.
1. Don't rub it in
I'ts important to remember not to rub it in, don't constantly bring up who is and isn't going. It will, obviously, make the situation worse.
It's also important to realize that constantly bringing up how unfair it is, even if you are trying to side with them, will seem like you are being condescending, I strongly recommend avoiding that as a topic of discussion completely.
2. Don't isolate them, don't isolate yourselves
Sure maybe there is a bit of friction between you at the moment, but sitting in two separate groups at lunch or at other social gatherings your work may have will only perpetuate the divide. Be sure to mingle, be sure to be part of the team.
3. If asked, don't engage
If people do try to engage you in this type of discussion, then say that the decision is not yours to make, and that they should, kindly, direct all of their questions to the bosses.
This may seem a little cold at first, but really it's making sure you don't get yourself dragged into a discussion that turns into an argument. Engaging in this type of discussion is likely to make things worse
4. Where possibble continue with your job
Congratulations, on being good enough at your job that the bosses want to pick you as someone to spend their, limited!, money on to train up some more. It's a good thing to have happen in your career. And whilst you may not agree with their decision you would be silly to try and throw it away by arguing and not working as hard as you have been up until now.