Green’s Dictionary of Slang

join-boy n.

[? Fanakalo joyin, a contract + SE boy]

(S.Afr.) a newly recruited miner.

[SA]O. Walker Kaffirs are Lively 22: Half a dozen reasons can be given why tribal Natives become ‘join-boys’, as recruited miners are called.
[SA] informant in DSAE.
[SA]B. Godbold Autobiog 14: Groups of blanket-clad ‘join boys’ as they were called, walked for many days to railheads, to entrain for Johannesburg, for employment underground [DSAE].