Sometimes, not everyone can make it.

With group shots, sometimes it is impossible or impractical to get all subjects together. 
With some planning, it is possible to create group shots that seamlessly combine people who were there at difference times.
Here is an example of one group shot combination.
 

The original unretouched portrait from 2013 where 1 staff member had left and one was away.

The original unretouched portrait from 2013 where 1 staff member had left and one was away.

The unretouched portrait from 2015 with the new staff and the staff available.

The unretouched portrait from 2015 with the new staff and the staff available.

The final retouched image with the current staff.

The final retouched image with the current staff.