In Dummit and Foote Abstract ALgebra, they define
A subring of the ring $R$ is a subgroup of $R$ that is closed under multiplication.
I know that since we check for closure and multiplication and subtraction, it is actually closed under addition, additive inverse and multiplication.
I don't understand how being closed under addition and additive inverses is in the defintion, does subgroup imply a subgroup under addition, even though they didn't specify the group law for the subgroup? Are subgroups implicitly groups under addition when talking about rings?