Data Modelling From Plain English

1create table users       (id serial primary key, email text unique not null, name text);
2create table projects    (id serial primary key, name text not null,
3                          created_by int not null references users(id));
4 
5-- "members" was a verb with facts: role, when they joined, when they left
6create table memberships (id serial primary key,
7                          project_id int not null references projects(id),
8                          user_id    int not null references users(id),
9                          role       text not null,
10                          joined_at  timestamptz not null default now(),
11                          left_at    timestamptz);                -- null = current
12create unique index one_current_membership
13  on memberships (project_id, user_id) where left_at is null;   -- history allowed
14 
15-- "invite" was a verb with a lifecycle
16create table invitations (id serial primary key,
17                          project_id int not null references projects(id),
18                          invited_by int not null references users(id),
19                          email      text not null,                -- may not be a user yet
20                          status     text not null default 'pending',
21                          sent_at    timestamptz not null default now(),
22                          responded_at timestamptz);

Two of the four tables came from verbs. email on invitations, not user_id, came from an unknown that became a question. Nothing here is indexed for a query, because no query exists yet; that is the handover.