JSON-LD for an artist that is a group, correctly includes schema to show who the members of that group are. But the reverse relationship is omitted, that is, an artist who is a "member of" several groups does include JSON-LD "memberOf" to specify this relationship. For example@
Apollo Saxophone Quartet relationship
Correctly includes http://schema.org/member for each band member
Band member Andy Scott relationship
Although it shows on the page that he is a member of several bands, including Apollo Saxophone Quartet,
...there is no JSON-LD http://schema.org/memberOf showing that he is a member of these groups.
I think this relationship is important, as it connects artists to their bands and groups, and does not need to be inferred by visiting several other pages.
Since JSON is already well implemented, hopefully this addition would not be too onerous.