When sending other currencies (eg. Bitstamp USD) to someone, what will they receive?
When you have an "other currency" setup in XUMM, and you send a transaction in this currency to someone, the currency will be transferred as:
If the recipient owns the same "other currency": it will be sent as is
If the recipient does NOT own the same "other currency": you will send (spend) the "other currency", and the recipient will receive the equivalent in XRP, based on the available conversion rate on the XRP decentralized exchange.
You can send other currencies by selecting the currency when manually sending funds using the Send button, or by tapping the currency from the home screen and pressing Send.
You can easily add currencies by tapping the Add asset button on the home screen. This is a curated list with trustworthy assets. You can add any other asset using a tool like xrptoolkit.com / xrpl.services and sign with XUMM.