There are two ways to get more personnel: 
  1. Start a hiring phase in the corresponding station.
  2. Buy 1 personnel for 5 coins each.
The advantage of buying personnel with coins is, that it is instant. If you start a hiring phase, you will get a new staff member every night, as long as that hiring phase is active.
1-day hiring phase: 1 new staff member
2-day hiring phase: 2 new staff members
3-day hiring phase: 3 new staff members
Automatically: any number of staff members up to the maximum of the station