If you come to Norway from a warmer climate country (say Israel, Italy, Spain, or even the UK), lunches can be quite a culture shock.
It's oh so quiet.
The first time I had lunch in Norway, in a company setting, I was extremely uncomfortable. As if I had farted and everyone noticed. Everyone was just quiet. Very little talking. Everyone was just... eating their very small sandwich, with milk perhaps reading something, and then going back to work.
I'm used to eating hot lunches in a loud environment. This was like eating in a surgery room while an operation is taking place.
Where did everyone go?
The norwegian way of being is very much "to each his own". That means that you are expected to "provide" for yourself, and generally speaking no one will wait for you to join them to lunch.
If you attend a BBQ, you will notice that everyone brings their own personal things, not to share. Be sure to always bring things for yourself. Don't count on their being a "common" area for food. So don't just bring drinks and rely on others to bring food.
If you are in your new job , you might be sitting at your desk, hunched over doing something, when you suddenly realize when you look up that there is nobody else around you anymore. It is 11.10 AM, and everybody around you at lunch already. None even thought of inviting you to go with them.
That's not entirely true. If the colleagues are extremely compassionate to foreigners, there is a small chance that someone might ask you if you will be joining them for lunch. Expect that to happen about 10% of the time.
If no one invites you it is easy to take things personally, but don't. It really is nothing personal. Just the way things are for everyone else too.
Aaannd... he's gone.
When someone does invite you to go to lunch with them at the cafeteria, don't expect them to do silly things such as hang around and wait 30 seconds for you while you lock your computer, finish that last word in the email, or tie your shoes. They'll very likely just go, and expect you to "meet them there". All this of course, goes without saying.
Literally, they do not tell you that.
When that happens, go and find them at the cafeteria, and hopefully there'll be some room left for you at the table. If not, that's OK. Find a table and wave to the next one you recognize who's looking for a place to sit. Eventually you'll find someone to sit with you and eat quietly.
You didn't do anything wrong. People are just used to either go in a group of people they already know, and they are just the way they are. Get used to eat, and don't feel frustrated. Try not to, at least. Try to catch the social queues, and see when everyone is leaving. leave with them. Just go with the flow. People are nice and talkative you initiate the conversation. Oh, and learn some norwegian if you truly want to be part of things.