Might and Magic Book One: The Secret of the Inner Sanctum
At the beginning,nes version only has one character. But dos version has more people.
Is the nes version easier than dos? Can add more member at the beginning?

You should be able to add more characters at the inn.

There are a few differences between the versions. I haven't played either of them, but I have watched some videos, particularly those of the NES version, and I know of a couple of differences:
1. In the NES version, you need to be level 3 to leave the starting town; that is not the case in the DOS version.
2. Locust Plagues are deadly in the NES version, to the point where I consider it to not be fair. If you are playing the NES version and you see a Locust Plague, kill it before it has a chance to attack; failure to do so is game over. Seriously, they're that bad. In the DOS version, Locust Plagues are a lot more tame, as they can only do 10 damage per action, which is manageable. In the NES version, they do 255 damage, killing whoever gets hit; and when that happens, the round restarts (I believe), and if the Locust Plague is faster than your party (very likely), it will get another turn. The result is a game over.

Whether NES is easier than DOS is a question I can't answer, but it is a good idea to know about Locust Plagues, as that particular enemy makes the game unfair when encountered; if even just one gets a chance to attack, your entire party will get wiped out.

(The NES version also, to my understanding, has a bug that can transform items into other items, which can be exploited to get you an advantage or to get key items without actually having to earn them properly.)

Edit: If the Locust Plague is in the front row at the start of the battle, running away might be a good idea, (NES version)