It doesn't matter who buys the passes. Passes are always separate.
When buying the pass you must write the correct name and country of residence of the person that will use the pass.
But how can we stay together in the trains , talking about seats?
Reservations are totally separate. You can make them on the Interrail website (adding each traveller's pass number and last name) but there's 2€ extra fee per person per train. It quickly adds up!
Better use the railway companies websites: cheaper, no fees, sometimes seat selection with a map, etc.
Have a look at https://www.seat61.com/interrail-and-eurail-reservations.htm
For further questions mention route and travel date.