Mention other Instagrammers on a post and they'll be notified in their Activity.
If someone with a private profile mentions someone who isn't one of their approved followers on a photo or video, the person mentioned won't get a notification in the You section of Activity and they won't get a push notification.
If someone with a private profile leaves a comment on a photo that a public Instagrammer uploaded, their comment will be visible and their username will be clickable below that photo.
If someone with a private profile sends a photo/video to a specific group of people with Instagram Direct, only people who follow the private profile will be able to see the comments or mentions associated with that photo/video.
Reply to someone in a comment thread by mentioning them in your comment. They'll get a notification in their activity feed to view the comment.
To reply to someone on iOS:
- Go to the photo or post
- Tap any comment
- Tap Reply and add your comment
You can also press and hold any username in the comment thread to automatically start a comment mentioning that username.