Verb (5-dan, る, intransitive)
1. to be added to; to be appended
説明が加わったことで、余計に意味がわからなくなった。The added explanation made the meaning even harder to understand.
2. to join in (e.g. a group of friends); to participate
僕もそれに加わりたくてさ。I wanted to join in on that, too.
3. to increase (e.g. heat); to gain in (e.g. influence); to grow; to gather (speed)
日ごとに寒さが加わってきた。It has been getting colder day by day.
4. to be applied (e.g. heat, pressure); to be exerted
熱が加わると色が変わる。It changes color when heat is applied.