Undoubtedly the word should be accepted. If it were always written with a lower case b, I would tend to agree that it should be classed as common, though I would still have a nagging doubt about how long it will remain in common use (the word, I mean - I don't think the currency has ever been in common use, and perhaps never will).
But my impression from looking in the News on the Web corpus is that it is written Bitcoin at least as often as bitcoin. The OED draft entry from 2014, that TRex noted, lists it as Bitcoin. (Some early citations write it as BitCoin, but that variant seems to have fallen by the wayside.) The online Oxford gives it a lower case b. Merriam-Webster allows both versions. It is apparently a proprietary term.
From what I can see the variant we can accept, with no upper case letter, is not used in the majority of cases, so I will add it as a rare word.