[itsme]

Until now I don't maintain composer and lyricist in MSP.
But if I would, I would put all in MSP's "Composers" field separated by | and specify with a prefix or postfix if it is composer or lyricist.
Thus "Custom Groups" stays available for other use cases.
Additionally: often just a list of names is specified on a score without mentioning who is composer and who is lyricist.

Example
on a score of "I Can't Give You Anything But Love" I find "J. McHugh & D. Fields"
I can look up e.g. in Wikipedia "by Jimmy McHugh (music) and Dorothy Fields (lyrics)"
I would store it in the "Composers" column of CSVs as "McHugh, Jimmy (music)|Fields, Dorothy (lyrics)"
MSP creates on CSV import two database entries in "Composers", one for "McHugh, Jimmy (music)" and one for "Fields, Dorothy (lyrics)" that are displayed as shown by attached screenshots