Is Hello everyone correct grammar to use in the beginning of a sentence?

"Hello everyone" is grammatically correct, but it's not a complete sentence. It's a phrase that acts as a greeting.

To make it a sentence, you need to add a verb and complete the thought. For example:

* Hello everyone, how are you today?

* Hello everyone, welcome to the meeting.

"Hello everyone" is often used in informal settings, especially when addressing a group.