I opened a cube the other day that had been created by someone else and was confounded for a short while about how it was formatted to display thousands, rather than units.
Select the Measures Dimension, then tight click and Edit Element Attributes. Select the element you want to set, click the format button, choose Custom and paste #,###,;(#,###,) into the Custom Style. This will then remove the last three digits from a number before the first comma from the right. Bingo!
This then works not only for thousands, but millions etc as well.