いっしょに (isshoni) means “together”

More examples

今日はいっしょに宿題をします。kyō wa i ssho ni shuku dai o shi ma su

Today, we do our homework together.

It shows that people take part in one activity or state as a group.

そろってsorotte: All members take part in the same activity or are in the same state as a group.

複数の人が同じことをするさま。fuku sū no hito ga ona ji ko to o su ru sa ma

The adverb 一緒に (isshoni) is formed from 一緒 (issho, "together") and に (ni), a particle that turns this shared state into the manner of an action.

A person can come before と一緒に (to isshoni), as in 友達と一緒に (tomodachi to isshoni), which describes doing something together with a friend.