Game cannot register to EOS or steam if user does not have it in the library. So if they will not have an option to play online unless he buys the game.
You can always do a check if there is a connection to the EOS by using IOnlineSubsystem::DoesInstanceExist(your_eos_name)
For steam, it is: IOnlineSubsystem::DoesInstanceExist("Steam") and that returns if you are connected to the Steam. Not sure about Epic. But I’m quite sure you can google it out 
Edit: I did some googling, and I think it’s “EOS”, but please confirm.
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