I'm assuming you're in the USA, since we're probably the only ones who have HOAs.
1. When you mash the shutter button, you automatically have copyright on your work. You don't have to register it.
1a. However, if you DO register it, it's way easier to go after infringers and thieves.
1b. If you put your logo or watermark on your images, you can separately go after infringers who remove your logo, even if you didn't register it.
2. Even without a contract, there's such a thing as implied license. You likely gave the HOA an implied license to use the images for whatever, either verbally or through casual emails. However, that implied license almost certainly didn't include license to transfer rights to another party.
3. The HOA reps almost certainly didn't understand that "You gave us the images, now they're ours, we can give them to anyone else, what's the big deal" is NOT how it works. Likely not intentionally malicious. The real estate company probably thinks they have the proper license to use the images.
4. You're gonna have to tread lightly here with the HOA. You likely won't be able to recover much in the way of monetary damages for unregistered images with an implied license and no contract. However, if you step on the wrong toes, the HOA reps can make your life miserable for as long as you live in that neighborhood. That's not a blacklist I want to be on.
I don't know your relationship with the HOA rep. That's going to be the big deciding factor on whether it's worth it to pursue anything for this particular infringement.