“我们一起去海滩吧！” or “让我们一起去海滩”??? *** I asked a native speaker about this the other day and he told me that using "让" sounds "too serious". But then today when I asked another native speaker about this, he told me that it'd be just natural and fine to use "让" to express "Let's...". Could someone please explain to me about the usage of 让? Is it really natural to use 让 to invite/persuade other to do something?
“让我们一起去海滩” sounds more formal and literary to my ear. I would expect to hear it from a formal speech. It sounds like you suggest that we have something meaningful to deal with in the beach. It's like, 让我们一起去海滩来见证这一历史性的时刻(let's move to the beach to witness this historic moment!).
“我们一起去海滩吧！” is a day-to-day phrasing and more conversational. So, use it in a casual speech.
Basically, 让我们 is considered to be a literary and formal device, and 让 is casually and habitually left out in colloquial contexts.