Thank you, that's good for future reference
The good news is that it turns out I was only posting to myself within that group so they couldn't have seen the annotations.
It seems that only highlighting as opposed to annotating posts to yourself without the chance to post in a shared manner. To share a highlight with the group I need to annotate instead, and leave say a single space as a comment, in order to activate the option to choose between posting to myself or to the group.
Is there a way to share highlights only with the group?