"There was a bard with her, to whom Agamemnon had given strict orders on setting out for Troy, that he was to keep guard over his wife;"
Agamemnon gave orders to a serving person, probably a storyteller or singer. The word "bard" means poet or songster. So, this person was given charge over Clytemnestra until Agamemnon's return.
However, "Aegisthus carried this bard off to a desert island and left him there for crows and seagulls to batten upon—after which she went willingly enough to the house of Aegisthus." Aegisthus had different plans and basically removed the bard who was watching out for Clytemnestra and took her off to his own house after he courted her with incessant flattery.
You will find this in the middle of book 3. But, you will not find it in any summary materials, so you have to read the text to find it.