Yes, it happened with just two people—both relatives. Perhaps it happened with others when I was very young, but I don't remember, and certainly not while they were actually watching.
The first was my stepsister, who was older than me. I remember she was often around when I was between 5 and 11 (she was eight years my senior), but as I entered puberty, she was there less and less. The last time was when I was 13; several years had passed since she had last witnessed it...
The other was my cousin, who was seven years younger than me. She and my aunt were often at my house, and when she was very little, she was always around while the procedures were being done. I’m certain she was present a few times; I clearly remember that when she was about four, my grandmother called her over because she was eventually going to have to undergo the same procedure, but she didn't want to because she was scared.
As the years went by, they were around the house less, but my cousin would still sometimes be present while I was getting an enema, even if not directly. She would only see me as I moved from the bedroom to the bathroom, though she indirectly witnessed the whole preparation process.
I remember her mother scolding her a few times for peeking (and I recall getting angry and asking for the door to be closed more securely), yet she would later brag to me that she had managed to see me... I think she did it partly out of curiosity, but mostly to be defiant and to annoy me.
This went on until she was about eight or nine years old.