Is it fine to post a paper to arxiv and later change the title and add authors? I have paper with very new contributions but it needs at least one month to finish perfectly. I am afraid that someone eat my launch and publish on the area. My advisor suggests adding the paper to Arxiv. but the title may change later. is it okay?

Also, I have just checked that changing authors when replacing a paper is technically enabled. Is it okay to add authors later?
Arxiv is just a preprinting service which is [generally] non peer-reviewed. The whole point of publishing there is to just put your idea out there (it gives you the copyrights) and to get feedback from your peers who are eager enough to read your draft. It is very common practice to update the title and author list for final publication in a journal. I have done it personally without any problem. That being said, all authors, who will be on the final publication, should be included in the arxiv submission, and not just the first/submitting author.