Dinner invite etiquette

Detroit

If I invite anyone to anything I tell them exactly what they're invited to and who else is invited, and so do my friends.

As to whether it's odd, that depends on what you're used to with these people. If this was a regular get together, just you and them, maybe a bit strange to invite others without telling you. But otherwise, they may have assumed you'd expect others there, or that it wouldn't matter.

Primrose

Yes I would tell people out of courtesy if they weren't the only guests. Just imagine what a tense evening you would have if your group included other people with whom they might have fallen out and you were unaware of it!