I am part of the team working on building a new payroll system.
I am involved in designing and building a new payroll system.
「I am building a payroll system」は100%通じますよ^^。英語ネイティブは「payroll system」や「payroll software」という言い方を使います。
他の言い方のバリエーションもあります。
I am part of the team working on building a new payroll system.
I am involved in designing and building a new payroll system.
I am developing new payroll software for our company.