Ch. 34 of [House Practice: A Guide to the Rules, Precedents and Procedures of the House][1] explains that "The general practice for election of Speaker begins with nominations from each party caucus followed by a viva voce vote of the Members-elect". We know who those people are; one of these days, they may make the transition to being "Members", after the Speaker is chosen and sworn in.


  [1]: https://www.govinfo.gov/content/pkg/GPO-HPRACTICE-108/html/GPO-HPRACTICE-108-35.htm