From Longman Dictionary of Contemporary Englishjoin up phrasal verb1 JOIN AN ORGANIZATIONto become a member of the army, navy, or air force2 British English to connect things, or to become connectedjoin something ↔ up The dots are joined up by a line. → join→ See Verb table
Examples from the Corpus
join up• Most of the boys went to town to join up.• My dad joined up at the beginning of the war.• When her brothers joined up in 1914, she took over the running of the business. 