From just an internet search:
Although the Constitution does not expressly authorize Congress to issue subpoenas, the Supreme Court has stated that the authority to subpoena is an "indispensable ingredient" of Congress' legislative power. Eastland v. United States Servicemen's Fund, 421 U.S. 491, 505 (1975).
and...
Contempt of Congress is the act of obstructing the work of the United States Congress or one of its committees . Historically, the bribery of a U.S. Senator or U.S. Representative was considered contempt of Congress. In modern times, contempt of Congress has generally applied to the refusal to comply with a subpoena issued by a Congressional committee or subcommittee —usually seeking to compel either testimony or the production of requested documents.
So even though its not a specified power in the constitution, it seems there is some legal precedent for congress to compel testimony. It seems to me that the only reason they should have to compel testimony is because the DoJ is NOT DOING ITS JOB, which would be investigating and charging folks under the law. Sad shape we are in.
Can someone please wake Sessions up, as he has turned out to be a nightmare.