If you're inviting colleagues who you might not know well over to your house, it's well and good to say stuff like "we have a dog" in the e-mail. This kind of generic information may be important to people who are allergic to dogs.

What's even more useful to more people is if you specify whether your dog is old and sedentary, or one of the crazy kind of hyperactive creatures that will jump onto and bark at random strangers as soon as they come inside...