啦 form a greeting
Please use 啦 to form a greeting in the following situations:

Example 1: 朋友病了一礼拜以后, 第一次来上课。--- 你好啦?
Example 2: 看见朋友开着一辆新车来上班。 (买新车) -- 你买新车啦?

一进门看见弟弟的朋友要回家。(不坐)
I don't know how to use 不坐 to greet him....
Maybe 你不坐电梯去啦? But 啦 indicates some kind of change.. but there's no change in "你不坐电梯去啦?" ... except if I asked 你这次不坐电梯去啦? but I wouldn't yet know if he planned to use the elevator so this question/greeting sounds strange too.

If I had 呆 (stay ) as an option, I'd ask him 你不呆啦?
"你不坐啦?" sounds strange to me, because why would you ask someone if he's not sitting (anymore) while meeting him at the door.

What would be a correct answer?

Question

啦 form a greeting
Please use 啦 to form a greeting in the following situations:

Example 1: 朋友病了一礼拜以后, 第一次来上课。---> 你好啦?
Example 2: 看见朋友开着一辆新车来上班。 (买新车) --> 你买新车啦?

一进门看见弟弟的朋友要回家。(不坐) 
I don't know how to use 不坐 to greet him....
Maybe 你不坐电梯去啦? But 啦 indicates some kind of change.. but there's no change in "你不坐电梯去啦?" ... except if I asked 你这次不坐电梯去啦? but I wouldn't yet know if he planned to use the elevator so this question/greeting sounds strange too.

If I had 呆 (stay ) as an option, I'd ask him 你不呆啦?
"你不坐啦?" sounds strange to me, because why would you ask someone if he's not sitting (anymore) while meeting him at the door.

What would be a correct answer?

Answer

你不坐啦？

Answer

1: 朋友病了一礼拜以后, 第一次来上课。---> 你好啦?
 2: 看见朋友开着一辆新车来上班。 (买新车) --> 你买新车啦?
3.一进门看见弟弟的朋友要回家。-->你不等电梯啦？你要走楼梯吗？或  你不再多呆会啦？这就回去了吗？

Answer

啦只是有语气词，句子的意思是由前面的文字决定的。你会啦？

Answer

I'll say "你不再坐一会儿啦？". "坐" here means to stay. Sometimes we can use "坐" to mean "have a visit". So "我要去我姑家坐一会儿" means "I'm going to spend some time at my aunt's place".