To Be Added To
To Join In
intransitive verb, godan verb
This word consists of kanji with hiragana attached. Because the hiragana ends with an う sound, you know this word is a verb.
The kanji 加 means add, and this verb means to be added to or to join in.
It is an intransitive verb, like most verbs ending in わる. That means that it happens by itself, or at least the cause of the adding is not you. Something gets added to something else, perhaps by magic?
くわわる
Since this word has okurigana (hiragana attached to the kanji) you know that it's probably going to be the kun'yomi reading.
Lucky for you, the kanji readings for transitive and intransitive verbs are almost always the same. Once you learn one, you know them both! This reading is the same as 加える. To help you distinguish between the two, remember that most verbs ending in わる are intransitive.
もうすぐ新しいキャラクターが加わるよ！
A new character will be added soon!
サルがバレボールの試合に加わった。
A monkey joined in the volleyball match.
材料の一千万円に、さらに広告のコストが加わる。
The cost of advertising will be added to the ten million yen for the ingredients.