As general wedding etiquette goes, guests just know that immediate family sit in the front row.
If you want the celebrant to oversee this then you should ensure clear communication around it.
I generally ask my couples about this very subject in my wedding workshops prior to the day so we can iron out every element of the ceremony, not missing a thing.
If the couple seem confused about it I give them options.
A) Engage 1 or 2 people (maybe the groomsmen or family member from each side) to meet & greet guests and, if necessary, usher people to their seats. This adds a lovely touch and makes guests feel special.
B) The Celebrant can seek these people out, with the help of the groom, introduce themselves and guide them to their seats. Generally we haven't met all of the parents/immediate family/grandparents etc before the ceremony so this can get a little tricky.
C) The Celebrant can make a general announcement if the couple have requested it. Choose the wording carefully.
D) Signs on chairs - Done right, they can look quite lovely and add / enhance the decorative side of the ceremony.
E) Some couples opt to let it 'sort itself out'
I hope you find this info helpful
Wishing you an enjoyable journey and an extra Special Wedding Day. XO