It depends. You are not required to pay non-exempt employees (those paid by the hour and entitled to overtime) for time spent at company functions when attendance is completely voluntary, no work is performed, the event occurs outside of regular working hours, and the event is not job-related. If your “welcome back” party meets these conditions, no pay is needed for non-exempt employees who attend.
Exempt employees (those not entitled to overtime) do not need to be paid extra for time spent attending a company function. They should be paid their usual salary whether or not they attend the party.