I think in an office setting like this, it's no big deal, nor is it an etiquette breach, to just ask if you can come along. It happens all the time in my office culture. Someone says they are headed up the street for a snack and one of us will just pop up and ask if we can go.
In our office, if lunch is something personal, you just don't announce you are going, you simply leave quietly. However, announcing you are headed to XYZ location usually means that others are welcome to join you. Announcing you stepping out for lunch and you'll be back at a certain time signals that you are not open to company.
I used to be really shy about joining people for this type of thing, but I gradually realized how casual it was. You can also start your own precedent. I will usually announce, "Coffee run. Anyone coming?" You can do that too. If you are more extroverted towards others, it will make these things more natural to you. =)