Hey there, 嗨 has two meanings. The first, and I believe the original one is a direct translation of "hi" from English. The second one, which is very popular at the moment in China, is like "excited" or "buzzing" or "having a great time in a place with an energetic atmosphere". It comes from the English for “high", as in the feeling after you take drugs high, but this is not what it means in Chinese, it just means that everybody was excited and buzzing and having a great time. I heard this all the time in China this summer. 起來 here just indicated that the process is starting. Everybody was starting to get really buzzing. Laurence

well,you are out ~~,it means have fun in China ,or take it easy.

get high and being fun

means just for fun ,cool

It from the english word high but in the two sentences you gave it more like cheer up .