Use 'join in' when you start doing something with a group already engaged in the activity.

Group Activities

'Join in' suggests a spontaneous or unplanned decision to participate in an activity.

Spontaneous Participation

Use 'join in' when you want to describe adding your voice to a group already singing or chanting.

'Join in' carries a sense of unity and common purpose, especially in group singing or chanting.

Use 'join in' to indicate that you are adding your support to someone else's opinion or action.

'Join in' can be used when you start contributing to a conversation already in progress.