The Fifth Amendment privilege against self-incrimination is personal. That means that corporations, for example, can’t assert it. It also means that you can’t invoke the privilege in order to refrain from incriminating someone other than yourself.
The Supreme Court has limited the Fifth Amendment to situations where the person claiming the privilege is subject to “physical or moral compulsion.” (Fisher v. United States, 425 U.S. 391 (1976).) When the government forces someone other than the privilege holder to divulge information about the privilege holder, the requisite “compulsion” typically hasn’t occurred.
Example: The government sent a summons to a taxpayer’s accountant that required production of the taxpayer’s own records in the accountant’s possession. The Fifth Amendment was of no help to the taxpayer because she—as distinguished from her accountant—wasn’t compelled to do anything. (Couch v. United States, 409 U.S. 322 (1973).)
Note, however, that an attorney may be able to assert the privilege on behalf of a client where, for instance, the government seeks documents that it wouldn’t have been able to get from the client, but which the client disclosed to the lawyer in order to obtain legal advice. (Fisher, supra.) And in some circumstances, people may be considered to be in constructive possession of documents they’ve entrusted to others, allowing invocation of the Fifth even though they don’t physically have the documents. (United States v. Hershenow, 680 F.2d 847 (1st Cir. 1982).)
To find out whether you can assert the Fifth on behalf of someone else, or someone else can assert it on your behalf, consult an experienced lawyer. For further reading on the privilege against self-incrimination, see Invoking Your Right to Remain Silent.