People who stand because there are no seats left.
It's usually not very comfortable to be a standee, especially if you wanted to sit down.
Examples:
The bus was so full that there were many standees.
The theater was crowded, so some people had to be standees.
She didn't like being a standee on the train.
The standees looked tired from standing for so long.
There were so many standees that it looked like a standing room only party.
The standees had to hold onto the bars to keep from falling over.