'Joint' in this context means that it is a joint committee of the House of Commons and the House of Lords which scrutinises proposed legislation for compliance with human rights, and conducts inquiries.
The Committee has six MPs and six Members of the Lords, a mixture of Conservative (5), Labour (4), LibDem (1), SNP (1) and crossbench / non-party aligned (1).
Harriet Harman was the chair but is retiring or has retired as an MP. She was formerly a solicitor and the Solicitor General for England and Wales, which is one of the government law officers.
Joanna Cherry's expertise and experience in the law would have obvious appeal - she's a QC and if I remember right, a former minister of justice in the Scottish government.