The arguments are comma-delimited. Each single argument is 2 space-delimited strings, where the first one is the group's title and the second one a colon delimited list of packages.
If you need to specify more than one group, or a group whose title contains a comma or a space character, using nested group elements is highly recommended.
E.g.,
group="XSLT_Packages org.apache.xalan.xslt*, XPath_Packages org.apache.xalan.xpath*"