I would appreciate it if those who can, would stay on here for another night or so.

= I would appreciate it if those who can stay on here for another night or so would stay on here for another night or so.

if从句中的主语带了定语从句who can...。由于定语从句中的can后面的部分和would后面的部分完全相同，故定语从句can后部分蒙后省略。逗号表示those后的定语从句有省略，此处停顿，听话人则会将those和would stay on...连起来理解，避免情态动词can和would直接连接可能导致的结构混乱。

能在这里继续呆一个晚上的人如果愿意继续呆一晚，我将不胜感激。