Ageru is used to decribe situations in which
a.) you give something to another person who is equal or higher than you in status, or
b.) another person gives something to another person equal or higher in status:
By way of example, let's consider a few situations in which Masako receives a ring (a diamond ring, perhaps?) from several sources:
Example 1: I give Masako a ring:
Masako-san ni yubiwa o agemashita.
I gave Masako a ring.
Example 2: John gives Masako a ring:
Jon-san wa Masako-san ni yubiwa o agemashita.
John gave Masako a ring.