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Can one use ことにする to express the idea "A decides that B will do X"?

Take the sentence 私は外へ出ることにした. The way I see it, 私 is the subject of both 出る
and した, with one usage omitted due to being understood. So in full the
sentence is

> 私は｛私が外へ出る｝ことにした。  
>  I decided to go outside.

Now what happens if the subjects are different? Can we say

> AはBが外へ出ることにした。  
>  A decided that B would go outside.

If this construction is not allowed, how else could you say this?
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grammar
Can one say "A decides that B will do X" with ことにする?
I think one of your assumptions is wrong. A nominalized verb phrase doesn't
necessarily need a subject (omitted or otherwise). Take a look at the
following sentence:

> [ **嘘をつくこと** ] は良くない。  
>  [ **Telling a lie** ] is not good.

I don't think there's a recoverable subject in the verb phrase 嘘をつく. I think
it's more like "telling a lie" or "to tell a lie" than "[someone] tells a
lie". And I think the same thing is true about your example:

> 私は [ **外に出ること** ] にした  
>  _As-for me, [ **going outside** ] decided-to_  
>  = I decided [ **to go outside** ].

The nominalized verb phrase [ **外に出ること** ] talks about the idea of going
outside, but it doesn't specify an actor via a subject or anything else.
There's no subject to recover here.

The topic 私 connects to the matrix verb した, not the embedded verb 出る. Usually
a topic splits things into two parts:

> 私は ｜ 外に出ることにした  
>  topicは ｜ _comment_

This is called a _topic-comment_ structure: the comment on the right tells us
something about the topic on the left. Essentially, the topic connects with
the entire verb phrase on the right half, the head of which is した. And that's
why it can't connect with 出る, which is stuck inside a relative clause.

If you'd like to say you decided to _make_ someone go outside, you can do so
by getting rid of the intransitive 出る and putting the transitive (causative)
出す in its place (as jovanni suggested in [this
comment](https://japanese.stackexchange.com/questions/13106/can-one-say-a-
decides-that-b-will-do-x-
with-%E3%81%93%E3%81%A8%E3%81%AB%E3%81%99%E3%82%8B#comment28108_13106)). This
makes an を-role available inside the nominalized verb phrase:

> 私は [ **snailboatを外に出すこと** ] にした  
>  _As-for me, [ **making snailboat go outside** ] decided-to_  
>  = I decided [ **to make snailboat go outside** ].

But you can't directly say "A decided that B will do X", only "A decided to
make B do X". The ことにする idiom doesn't have a separate role for B to fill.

* * *

(Note: in the above, decided-to really corresponds in meaning to ことにした, not
just にした. Even though ことにする isn't a single constituent, it _is_ a single
idiom, so you can't really divide it up when talking about what it means.)
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answer
I would like to add some intricate points to snailboat’s answer.

Snailboat is right in that in general, verbs in Japanese do not have to have a
subject, even an implied one. Also it is right is to say that we cannot use
～ことにする to state someone decides that someone else will do something. ～ことにする is
used when someone decides what _he/she_ will do.

However, there are cases where BがXすることにする sounds fine to me.

As you might be aware, the sentence

> (1) 私は私が外へ出ることにした。

is unnatural. However, if there is some context where who goes outside is
important, then it makes sense to say:

> (2) 私は自分が外へ出ることにした。 I decided that _I_ would go outside.

For some reason, it is strange to repeat the topic and the subject as in (1)
even when who goes outside is important.

As another example, suppose that a team of athletes discussed the running
order in an upcoming relay race. Then it is fine to say:

> (3) 話し合いの結果、康介がアンカーを務めることにした。 As a result of the discussion, they decided
> that Kosuke would run as the anchor.

The subject of 務める is 康介 and it is explicitly stated. The semantic subject of
務めることにする is the team, and it is not explicitly stated but implied by the
context.

(I think that there is another interpretation of (3) depending on the context.
See the remark after sentence (5) below.)

If we say

> (4) 話し合いの結果、康介にアンカーを務めさせることにした。

then I think that it means that Kosuke was not one of those who decided it.
Probably he was not at the discussion, or he participated in the discussion
but he did not agree to the decision.

By the way, if we change が to は in (3) and say

> (5) 話し合いの結果、康介はアンカーを務めることにした。

then it is more likely to mean “As a result of the discussion, Kosuke decided
to run as the anchor.” That is, 康介は specifies the topic of the whole sentence
rather than just the subject of the アンカーを務める part.

It is also possible to interpret 康介が in (3) as the subject of the whole
アンカーを務めることにした part. In this case, (3) means roughly the same thing as (5),
with an emphasis on the subject: “As a result of the discussion, it was Kosuke
who decided to run as the anchor.” Therefore sentence (3) may be ambiguous
depending on the context. But given that it was a result of the discussion, I
think that it is more natural to interpret that the team decided it unless the
context implies otherwise.
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