From this Apple support document:

Apple Card Family lets you co-own your Apple Card account with one member of your Family Sharing group. You can share your credit line with a co-owner and build credit together as equals. You can also share your Apple Card with members of your Family Sharing group, including teens and adults. Everyone on the shared account can use Apple Card and view their spending. Account owners and co-owners can see a participant’s activity, set transaction limits, and more. And there’s a single monthly bill.

Co-ownership involves risk! From the footnote:

Each co-owner is individually liable for all balances on the co-owned Apple Card including amounts due on the existing co-owner’s account before the accounts are merged. Each co-owner will be reported to credit bureaus as an owner on the account. In addition, co-owners will have full visibility into all account activity and each co-owner is responsible for the other co-owner’s instructions or requests.

As to participants, they are not liable for their charges.

Coming July 2021: You will be able to invite another Apple Card owner to combine credit limits and form one co-owned account.

You can have up to 6 people (yourself and 5 others) on an account.

Apple makes it easy to add co-owners and participants. Watch the videos below (very short) for details.