When does the phrase "Some of us..." include the speaker?

Some of us is an indexical expression, which means that it picks out different people in different contexts. Sometimes it includes the speaker, sometimes it does not.

Suppose the speaker is a member of a book club that is debating its next book. Suppose the speaker and two others want to read Moby Dick, but another three members don't. If the speaker utters:

"Some of us want to read Moby Dick."

then he includes himself when using "some of us."

Now suppose the speaker is a member of a book club that is debating its next book, and that three members want to read Moby Dick, but the speaker himself doesn't. Now suppose he is talking to another member who has not yet decided which book they want to read. When the speaker utters:

"Some of us want to read Moby Dick."

he does not include himself when using "some of us."

It is probably the case that when speakers use "some of us" they overwhelmingly more often include themselves than not. (But it is virtually impossible to check this conjecture since corpora do not encode the requisite semantic information.)

Now for some general diagnostics.

When the speaker does not want to include himself, he will often place an emphasis on the "some." For example,

"Some of us want to read a terrible book."

Further, examples where the speaker does not include himself easily take parenthetical remarks which make it clear that he does not include himself. For example,

"Some of us want to read a terrible book (but I don't)."