How to record HDMI video output?
I would use three pieces of hardware for this task.


A recording laptop.
A device that allows you to input the HDMI to the recording laptop, aka, a frame grabber.
An HDMI spliter.


A recording laptop

Any old laptop will do, really. It doesn't need super specs, like dedicated graphics, but you certainly want at least a mid-range machine. Like an Intel i5 or better, 8 Gig's RAM, fast HDD (SSD would be great, but not necessary). Software wise, anything that can see the generic devices will work. Pro software like Vegas Pro or Adobe Primere if you have it works great. There's also Wirecast and vMix if you have webcast requirements.

A video frame grabber

I've had pretty good success with the DVI2USB3.0 Frame Grabber by Epiphan. Use that with a DVI to HDMI converter and it will work just fine. It also allows audio capture. I especially like its very small size, solid construction, and that it's USB powered. It will work on USB 2.0, but you will get limited bandwidth in the form of limited framerates per the source resolution (I often get about 45 fps at 1024x768 on USB 2.0). It's a bit on the pricey side, but Epiphan has very good customer service. I know other options are available, but I have no experience with them. About 5 years ago I decided against Black Magic products because of the mixed reviews on Amazon, but that may have gotten better. Other items that are like $10,000 are not the kind of equipment you are looking for. I've never actually seen one of those being used for more than what a $1,000 in hardware can do. Basically, if you don't know why it costs so much, you don't need it.

HDMI video splitter

There are plenty available on the market (Google image search). They're pretty simple. They have a single HDMI input and at least two HDMI outputs, which are both exact duplicates of the input signal. Make sure you get one that is powered because the ones that aren't reduce signal strength and it often shows, especially on projectors (at least, this is true for the VGA splitters). If you intend to audio record over HDMI make sure the splitter you get can handle that.

Setup


Take your source HDMI and run it to the video splitter.
Run an HDMI from an output on the splitter to the projector.
Run an HDMI from an output on the splitter to the Epiphan frame grabber via a DVI to HDMI converter.
Plug the Epiphan USB into the recording laptop.


Alternative, VGA-based setup

I would actually recommend this setup if this is in a professional conference setting and there is a professional audio/visual crew working it. I also recommend this setup if any of the HDMI runs are farther than 30 feet. HDMI just can't push very far and VGA can easily do 100 feet.

If this is a professional setup and you're just hooking in to what they already have, then the projector is not yours and you don't know what to expect. Chances are very high that projector will be gathering it's feed via VGA, not HDMI (you really should call the AV company to find out though).

So instead of using HDMI from the source laptop, use the VGA out (if it has one) or convert it to VGA with an HDMI to VGA dongle. The setup is exactly the same as above except everything HDMI is VGA now, including the VGA video splitter and the DVI to VGA adapter. For audio, just plug into the recording laptop's mic input port. As a warning, you might need some kind of audio cleanup when gathering audio this way. I regularly use the Whirlwind line isolator and the Dsan laptop sound port.

Source

I work for a company that does this all the time. If you need professional help with content production you can find information in my profile.