Installing maps from outside sources is a violation of Microsoft's Terms of Use. You may join a friend's game to play on maps other than your own, but sharing map files is not allowed.
Source (From the Microsoft Terms of Use):
"You may not modify, copy, distribute, transmit, display, perform, reproduce, publish, license, create derivative works from, transfer, or sell any information, software, products or services obtained from the Services."
Installing maps from outside sources is a violation of Microsoft's Terms of Use. You may join a friend's game to play on maps other than your own, but sharing map files is not allowed.
Source (From the Microsoft Terms of Use):
"You may not modify, copy, distribute, transmit, display, perform, reproduce, publish, license, create derivative works from, transfer, or sell any information, software, products or services obtained from the Services."