## ADDED Requirements

### Requirement: Shop group-buying activities
Shop users SHALL manage only their timed group-buying activities, each bound to one owned SKU with localized display content, fixed minor-unit group price, required paid-member count of at least two, group lifetime, and a per-order activity quantity of 1. The service SHALL reject a SKU that already has an overlapping active flash-sale item.

#### Scenario: invalid participant target
- **WHEN** a merchant configures a group activity requiring fewer than two paid members
- **THEN** the API rejects the activity

#### Scenario: overlapping flash-sale SKU rejected
- **WHEN** a merchant publishes a group activity for a SKU that already has an overlapping active flash-sale item
- **THEN** the API rejects the activity

### Requirement: Customer group discovery and intent
Customers SHALL discover active group-buying activities and open groups. A customer MAY initiate checkout by opening a new group or selecting an open eligible group for exactly one activity SKU at quantity 1.

#### Scenario: join an open group
- **WHEN** a shopper selects an active open group and checks out its activity SKU
- **THEN** the resulting pending-payment order identifies that group and snapshots the group price

### Requirement: Group membership and lifecycle
Payment SHALL atomically claim a paid membership in an open unexpired group. A group SHALL become successful exactly when paid member count reaches its required count; it SHALL become expired when its lifetime ends before success; it SHALL become cancelled when its opening pending-payment order is cancelled while paid member count is zero. A full, expired, or cancelled group SHALL reject payment without changing order state.

#### Scenario: concurrent final seat
- **WHEN** two pending group orders attempt payment for one final seat concurrently
- **THEN** exactly one payment succeeds and the group becomes successful

#### Scenario: expired paid group
- **WHEN** an open group expires before reaching its required count
- **THEN** it is marked expired and its paid orders remain identifiable for a later refund flow

#### Scenario: unpaid opener cancel closes an empty group
- **WHEN** the opening pending-payment order is cancelled and the group still has zero paid members
- **THEN** the group becomes cancelled, SKU stock is restored, and later join or payment for that group returns 409