If you are trying to add a new user with a different domain you will receive an alert that the user has a different domain to your firm and that they must be assigned read only access.
Accounting subscription packages have restricted use for users within your firm. These subscriptions are a charged at a lower rate than our corporate plans.
Assuming the user is are engaged by you rather than being your client, you can set up an email account at your domain for your contractor and invite them as a user with this email address.
Alternatively, upgrading to one of our Corporate Plans will unlock the ability to invite users with email addresses from other domains.