There currently isn't a way to do this. With expression language you are either referencing a function (listed here https://nifi.apache.org/docs/nifi-docs/html/expression-language-guide.html) or variable. The variables could be flow file attributes, system properties passed through bootstrap.conf, or variables from the variable registry (https://nifi.apache.org/docs/nifi-docs/html/administration-guide.html#custom_properties). I don't know of a reason why there couldn't be an EL function like ${processGroupName()} which returned the name of the enclosing process group. It just hasn't come up before.
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