Making a location index

I know this has to be basic but it is eluding me.

I need to create a reference index for 15 point feature classes based on the Map Grid polygon layer.

Normally we perform a spatial join to the polygon layer then a table join back to the original point layer, calculate the index layer in the point feature then remove the join and get rid of the original spatial join.

I would think there is a way to update an index field for a number of point features based on what polygon they are inside of.