我参加了志愿者活动用英语怎么说

"I participated in a volunteer activity" 或者 "I took part in a volunteer event" 是表达参加志愿者活动的常用英文表达。
来源：participate 是参与的意思，take part 是参加的意思，volunteer 是志愿者的意思，activity 和 event 是活动的意思。
例句：
I participated in a volunteer program at the local homeless shelter.
我参加了当地流浪汉收容所的志愿者项目。
I took part in a volunteer event to clean up the beach last weekend.
我上周末参加了一个清理海滩的志愿者活动。
I signed up to be a volunteer at the community center.
我登记成为社区中心的志愿者。