Challenges

You control this per challenge with a Visibility setting: public (anyone can find and view it), followers-only (only people who follow you), or private (only you, plus anyone you specifically invite).

From a private or invite-only challenge's access settings, search for a person and send them an invite. They get a notification with Accept or Decline actions, and accepting gives them view (and, depending on the challenge, join) access even though the challenge itself stays private to everyone else.

It depends on the challenge type. Daily challenges count how many distinct days you've checked in against how many days the challenge covers. Number challenges compare your running total (the sum of every increment you've logged) to the target you set. To-do challenges compare completed tasks to the total task count.

That depends on the challenge's visibility and who-can-join setting. A public challenge can generally be followed or joined by others as a shared challenge; a private one only opens up to people you've explicitly invited.

Nothing breaks — the day is simply left unchecked and the challenge keeps going. It does interrupt a streak if you're keeping track of one, but a missed day never fails or resets the whole challenge.

Yes — post progress updates, photos, or thoughts directly from a challenge, and they'll show up as posts on your profile and in the feed of people who follow you. Each post can link back to the challenge it's about.