"They climbed up on the table."
Translation:Elles sont montées sur la table.
Technically we don't know it's people. Could be a bunch of cats or bugs, just says "They" did it.
The sex of the noun controls the pronouns. In this case the "table" (feminine) is the noun in this sentence. If the sentence was "Jack climbed on the table and then he climbed down, then jack (masculine) would be the noun controlling the pronoun."
I have massive problems with this one too, and I may be wrong, but my understanding is:
ils (meaning they) and elles (meaning they) are used in the plural to talk about things, as well as about people or animals. Use ils for masculine nouns and elles for feminine nouns.
In this case "the table" is the noun, and thus makes the sentance femanine.