Traditionally, groom's parents have been expected to pay for certain parts of the wedding, such as the rehearsal dinner, the ring for the bride, and the flowers and bouquets for the wedding party--just to name a few.
Its the responsibility of the groom's parents to attend their guests and see to it that these guests / relatives are properly introduced to the bride's relatives. The groom's parents should give respect to the bride's family members thereby they can command respect for themselves.
The grooms parents are responsible for paying for the rehearsal dinner, the tux's for the bridal party and the honey moon. However with today's changes in the economy the groom is often helping to pay for the wedding.