To 及ぶ is "to reach" or "to amount to." The noun version is more like a grammar thing, as it means "as well as." Perhaps if you are reaching a certain point or certain amount, then you are adding things on. That's why "as well as" is the meaning of this one.
The reading comes from 及ぶ, so if you know that one you know that one as well as this one.
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