They have girlfriends. Each of the men has a girlfriend. The men died by accident. (This is a very odd thing to say) The men lost their phones by accident. ‘By accident’ is a set phrase but it would normally be quite callous to use it for something ending in death. For that you might say The men died in an accident. The men died in separate accidents. Also your question is posed poorly. The word ‘among’ is incorrect here. Also to understand how the language works, you should request clear examples, not limit yourself to weak artificial examples.

1 - They have girlfriends. 2 - Both of your sentences are the same! They are correct.