WARNING Though the numbers below match the PIN numbers on the published PDF they DO NOT match the numbers normally shown for the Pi. e.g. 5V is on 2 and 4 normally.
Normally I would send you to the great documentation site pinout.xyz for details BUT in this case, the only Pi-Supply board is the full sized hat for the Pi-Juice board.
Fortunately they do publish a schematic on GitHub BUT it is three years old, for version 1.1 of the PiJuiceTop and may not match your board.
Looking on that you can see the board documented (i.e. it may not match yours) uses:
See warning above on these numbers
Ground on pins 5,13,19,29,33,10,26,40. (odd given they are all common on the Pi board).
3v3 on 18
5v on 1,3
ID SCL and ID_SD on 27, 28 (to tell the Pi it is the UPS card)
SDA 1 / SCL 1 on 4,6
TBH - I would go back to the suppliers and ask for confirmation this is still valid.
A QUICK scan of the GUI source shows line 34 interrogating address 0x14 on I2C Bus 1 to see if anything is there and that matches the hardware diagram. I'll let you decode the core operating function PiJuice if you want more info :-)