How can I on board multiple sellers on my platform?

You can add sellers to your store either by admin panel or sellers can register themselves on your store.
To know more about how to board multiple sellers, please refer to the following link: https://help.storehippo.com/topic/managing-sellers

You can define commissions in the Sellers > Seller ledger rules section of StoreHippo admin panel. You can define commission on the product listing, product selling or order shipping. For more information, refer to the link: https://help.storehippo.com/topic/seller-ledger-rules