Had a bit of struggle to get this to work, logging the solution here so that it will be helpful to someone else. I am using Scala 2.9.1 and mybatis-scala-core 1.0.0.

val findOpenGroups = new SelectListBy[GroupIds,Group] {
def xsql = SELECT h.xxxxxx AS id,
h.xxxxxx AS launchedDate
FROM USA_xxxx_xxxx_xxxx h
WHERE h.new_status = 'Open'
AND h.id IN
<foreach item="item" collection="ids" open="(" separator="," close=")"> 
                {"#{item}"}
            </foreach> 
}

GroupIds is a simple class, just make sure you are using the java.util.List.

class GroupIds(val ids:java.util.List[Int]){}

UPDATE
Frank Martínez pointed out to me that you can also do {"item" ?} instead of {"#{item}"} but you will need to import org.mybatis.scala.mapping.Binding._ Thanks Frank.