Creaking Aluminum Group Chair  

Maybe just tighten the screws?

For aluminum-on-aluminum 'dry' lubricants, I like Sailkote McLube.

The brace is not held by screws, but by what appears to be unthreaded pins that were hammered into place. Don't see any easy way of tightening it.

Hmmm... Well, try the McLube then. Pricey, but great stuff for this and countless other applications. Just watch the overspray. It's incredibly slippery.

Edit: Yes, I now see that HM decided to use roll pins in place of set screws to fasten the back stretcher at some point. Thx for pointing out that tidbit!

I redid a set of six a few years ago and one chair’s stretcher bar as it sounds you describe had a split in the aluminum blade end right at the milled hole for the roll pin that sets it into the track. It too made a creak. It wasn’t visible until I tapped out the pins and looked at it. If you have the correct sized tool, they are very easy to tap out and put back in with the same tool. The stretcher bar ends have a curve so that you can gently slip it in and out of place. You’d have to tap out all four to remove it and look. I’ve ordered replacement parts for other Herman Miller items. It’s possible that that stretcher is purchasable if indeed you have a similar crack to the one I found. In our case it was opted to reinstall the damaged piece. I never heard that it eventually failed