DocumentReference serialization is type path in Firestore rules, forward slash paths are type path in "raw" form. Here's a utility for checking ownership

firestore.rules

function sentInvitation(invitationId) {
  return exists(/databases/$(database)/documents/accounts/$(request.auth.uid))
    && get(/databases/$(database)/documents/invitations/$(invitationId)).data.invitedBy == /databases/$(database)/documents/accounts/$(request.auth.uid)
}

// In production, wrapping exists and get in small utilities is worth tradeoff in code readability
// chaining this with utility that null-checks auth and auth.uid helps read rule errors in unit tests