Whilst creating a new macro for the macrocore library (mm_adduser2group) I received in my log: ERROR: Expected a comment or processing instruction.

The issue? Malformed XML (of course). I'd forgotten to add the <Metadata> tags. The correct code block would have been:

<UpdateMetadata><Reposid>$METAREPOSITORY</Reposid><Metadata><Person Id='&uuri'><IdentityGroups><IdentityGroup ObjRef='&guri' /></IdentityGroups></Person></Metadata><NS>SAS</NS><Flags>268435456</Flags></UpdateMetadata>

Code now fixed, and the SAS community has a new way to add users to groups metadata: