How do I add an OR range to a query in X++?

For multiple values on the same field, a comma-separated range works as an OR.

QueryBuildRange range;

range = qbds.addRange(fieldNum(CustTable, CustGroup));
range.value(queryValue('10') + ',' + queryValue('20'));

That matches group 10 or 20.

For more complicated OR logic across different fields, use a query expression. Those expressions can get ugly fast, so keep them as readable as you can.