Well I'm not sure how you have Soloist mapped....to a custom field or a UDTI. In the following action I mapped it to Custom 3. The following assumes a 'last name' is preceded by a space. It will place a last name (if any) from Artist and then from Soloist into the Grouping field. If none are found, the Grouping field will be set to empty.

Try this 🙂

Force Grouped Execution
Clear the Grouping field
Copy the Artist field to Variable 1
Find " " from the end in Variable 1, remove it and everything before, set state
if true
Add "\v1" to the end of the list in field Grouping delimiter ", "
endif
Copy the Custom 3 (Soloist) field to Variable 1
Find " " from the end in Variable 1, remove it and everything before, set state
if true
Add "\v1" to the end of the list in field Grouping delimiter ", "
endif

The above code could be enhanced to handle multiple Artists separated by the multi-value delimiter or comma or anything else.