好了 means suggestion. I don't know why you put “好不好”in the end .And when we use “一起”，the subject is always a plural.

The first answer is right，It should be e.g. 我们一起學習好了 好不好 我们一起玩好了 好不好 “好了”Be prepared to do something or ask someone to do something e.g 晚餐做好了，大家一起吃饭吧 dinner is ready, we all have dinner together. 你打车好了，那样比较节省时间 You can take a taxi, so you can save a lot of time

may i add you, i'm learning english. and i'm a Chinese.

In most cases, it means something has been done. Ex: This work has been done. 这项工作已经被做好了。 Ex: -- Are you ready? 你准备好了吗？-- Yes, I am ready. 是的，我准备好了。 Hope it makes sense.

“好了”意为对某事的建议。 for example ： It's cloudly ,so we should prepare an umbralla.现在是多云 我们还是准备把伞好了