Basic workflow

Create party using PUT /api/v1/party endpoint. It consumes username and password of administrator (which can't be restored). As the result it returns unique id of created party.
Create new users which have access to this party. Note that user belongs to specific party id and in scope of the specified party it must be unique.
Add players with their best in slot sets (probably by using ariyala links).
Add loot items if any.
By using PUT /api/v1/party/{partyId}/loot API find players which are better for the specified loot.
Add new loot item to the selected player.

Limitations

No limitations for the API so far.

For the most party utils service requires user to be authenticated. User permission can be one of get, post or admin.

admin permission means that the user is allowed to do anything, especially this permission is required to be able to add or modify users.
post permission is required to deal with the most POST API endpoints, but to be precise only endpoints which modifies party content require this permission.
get permission is required to have access to party.

admin permission includes any other permissions, post allows to perform get requests.