Thomson Reuters对此有个定义：An agreement joining a person as party to another agreement as if such person was an original party to such agreement. Joinder agreements are commonly used when new stockholders or LLC members receive equity and are made party to an existing stockholders' agreement or LLC agreement.

个人觉得翻译为“SPA加入协议”，但感觉差点意思； “SPA入伙协议” 似乎像加入了合伙组织， 要不理解为“SPA增员协议”。