As per section 18 of the Indian Partnership Act, 1932, partner to be agent of the firm i.e. legal position of a partner in a partnership firm is an agent.
The general rule is that an agent is not entitled to delegate his authority to another person without the consent of his principal. 'Delagatus non potest delegare' is the maxim which means that a person to whom authority has been given cannot delegate that authority to another.
Only partners being agents signing business agreements on behalf of the firm can bind the firm. No one else other than partners can bind the firm by such agreements. No difference whether registered or unregistered. An unregistered firm cannot sue on a contract with other party of the contract even though contract signed by a partner.