To extract sequences for particular read group id you can use Picard's SamToFastq and FilterSamReads. But you can't really specify specific coordinates to extract your reads. If you pretty adamant about using Picard java-API, you may have to use a combination of samtools view function and Picard's FilterSamReads. Otherwise you can do sth like:

samtools view chrA:X-Y Input.bam | grep "readGroupId" >  new.sam

There is no more picard-api. The java api used by picard tools is htsjdk.

Create a SAMReaderFactory:

SamReaderFactory srf=SamReaderFactory.make();
srf.validationStringency(ValidationStringency.LENIENT);

Open the SAM:

SamReader samReader = srf.open(bamFile);

Loop:

SAMRecordIterator iter= srf.iterator();
while(iter.hasNext())
{
    SAMRecord rec= iter.next();

Get the read group, get the sampleName:

SAMReadGroupRecord sgr=rec.getReadGroup();
String sampleName = (sgr!=null ?sgr.getSample() : null );