Technically, they do not become a member until the House recognizes them as a member once the new Congress convenes on January 5. And yes, technically, Nancy Pelosi could get the rest of the Democrats to refuse to seat any new Republicans. They have done it in the past and got away with it. Nothing is to stop them from doing it again en masse (except for Amendment II and the Fourth Branch of government, perhaps).
Read the history of Congress after the War of Yankee Agression. The Confederate states were considered “states in rebellion” and where not seated for the 13th, 14th & 15th Amendments until after they had been ratified. MS did not ratify the 13th until 1995.