This is the 9750th most frequent Japanese word.
薄まる
To become diluted or weaken.
In this sentence, '薄まる' is used to indicate that the flavor or concentration of the soup has become weaker or less intense.
スープが薄まったため、もう少し塩を足しました。
The soup became diluted, so I added a bit more salt.
Here, '薄まる' is employed figuratively to imply that an emotion, in this case anger, has reduced in intensity over time.
彼の怒りは時間が経つにつれて薄まった。
His anger lessened as time went by.
In this context, '薄まる' is used metaphorically to describe a potential reduction in the depth or intensity of interpersonal connections within a team due to increased membership.