Recruit; register (5)
I believe the answer is:
enrol
I believe this is a double definition.
'recruit' is the first definition.
(synonyms)
'register' is the second definition.
(enrolling is a kind of registering)
(Other definitions for enrol that I've seen before include "Sign up (member)" , "Enter" , "Loner (anag.)" , "Sign on, being a loner" , "Join as member" .)