It is not supported directly. The object type (-o) can only be on of source, target, transformation, mapping, mapplet, session, worklet, workflow, scheduler, session config, or task, s. ObjectExport Reference

However, you can export the Deployment Group based on a query. For example, if you define a query like this (or based on a lable) you should be able to do:

pmrep executequery -q test_group_export -u deployment_group
pmrep objectexport -i deployment_group -u deployment_group.xml

where deployment_group are object Ids and deployment_group.xml the resulting xml representation.