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This question on Reddit piqued my interest, and as of now doesn't have an adequate answer. The sentence in question is:
古見さんに友達が増えた. Komi-san made a new friend.
I know に is a very versatile case particle with many meanings, but what is the specific usage here? Any other example sentences with this type of に? How is the nuance different from 「古見さんの友達が増えた」 or 「古見さんは友達が増えた」?
It’s not much different from sentences like this.
駅前に店ができた。
This できる describes a change of state, a kind that puts something into existence somewhere. That somewhere is marked with に. This usage of に could be seen as basically the same as the standard usage of the particle in the sentence below, except the existence described will happen only as a result of the change described by the verb.
駅前に店がある。
増える in your sentence works in a similar manner. As a result of the described change, Komi-san has a new friend or friends, or they come into existence in Komi-san. In fact, this state can be described as below.
古味さんに（新しい）友達がいる。
(This usage of に is discussed here.)
増える doesn’t always work like that, though. For example, 人口が増える doesn’t describe a change that newly puts something into existence somewhere. It merely describes a change in some variable. So, the following sentence is a bit strange.
日本に人口が増えた。
I think the English translation is misleading here. Literal translation will be "Komi-san's friend(s) became more numerous", or even more literal "Friends became more numerous at/for/to Komi-san". So friend(s) are the actor and Komi-san is a recepient. As such she is marked with "ni". But of course she can be marked with "ha" as well.
It works similarly in Russian, so it's not peculiar to Japanese.
