Is it permissible to sit with smokers, whether at home or in public places? And is it obligatory to leave a place if a smoker is present in it?

Smoking is prohibited (haram (definition)), even though there is no explicit text stating so, due to the general principles of Sharia (definition). If you advise a smoker gently and he does not respond, then it is obligatory to leave his company, based on the words of the Almighty: "And He has already revealed to you in the Book that when you hear the verses of Allah disbelieved in and mocked at, then do not sit with them until they engage in another conversation. Indeed, in that case, you would be like them." This is unless it is in a workplace and you are compelled to stay.