While Chacun sa part was written with ease of use in mind, there's no harm in having a more detailed manual.
The most common way to use Chacun sa part is simple:
Chacun sa part is meant to be usable by groups of friends. In this context, a friend is someone with whom you share some involvement within a group (either as the creator of the group, the payer for one of its expenses, or one of the guests).
As we've seen, “friends” are registered automatically when you select “New…” when creating an expense or adding a guest to it. They'll be known to Chacun sa part, and you can select them again for future expenses. At this point, you may want to “invite” them from the “balances” tab. The invitation system generates a link that you can send to your actual friend; if they click on the link (or flash the QR-code), they'll be able to associate their account with that registered “friend”, which will give them access to the groups you have in common.
The group details pages on Chacun sa part also contain a link (and a QR-code) that allows for temporary access to a group. Be careful with that link: any one who gets it can access and modify the group and its expenses.
Chacun sa part knows of several languages and locations, which has two consequences. First, as a registered user, you can pick your favorite display language. Second, a group is associated with a given location, which allows for groups in different currencies; this alters the way amounts are displayed, but also the way they are stored within Chacun sa part and computed. Be careful if you decide to change the location for a group after you've registered expenses and/or paybacks.
Because some groups may have special conventions (such as “half rate for kids“), you can set the numbers of parts of a group to be fractional (by halves, thirds or quarters). Switching from one setting to another after expenses are registered will change the number of known parts in the simplest possible way while preserving existing ratios (and hence, balances).
Chacun sa part is meant to track expenses for groups of friends. It is expected that the persons involved act in good faith. If you share a group with someone, they'll be able to see and change the expenses and paybacks. This makes it easy to keep tabs as the holidays/parties/regular pub meetings go on, since everyone can register their own expense as soon as they make them, but there's no protection against malicious “friends”.
Note that a group (and its expenses/paybacks) is only accessible to people involved with it, so complete strangers shouldn't be able to come and disrupt your accounts if you haven't shared the group link.
Within a group of well-meaning friends, however, Chacun sa part is expected to be convenient and simple to use. I hope it fulfills that goal!