くわ加kuwaわるwaru
1.
To increase in number or amount by becoming part of an existing group or collection, often resulting in an enhancement of the overall quantity or intensity.
2.
To join in or participate as a new member in a group, activity, or conversation, thereby contributing to the collective effort or experience.
3.
To have an effect or influence on something else, indicating that an external factor is introduced that modifies or enhances a condition or situation.
Example:
これがごく最近私の蔵書に加わった本です。