Dummy equals 1 if has the same entry as in another row

You want an indicator variable for a being in the company at a given time. The approach of @Cybernike can be telescoped like this:

clear
input Year  str3 PersonID   Company
2010    a   1
2010    b   1
2010    c   2
2010    d   3
2010    e   1
2011    a   2
2011    b   1
2011    c   2
2011    d   3
2011    e   1
end

bysort Year Company : egen wanted = max(PersonID == "a") 

list, sepby(Year Company) 

     +------------------------------------+
     | Year   PersonID   Company   wanted |
     |------------------------------------|
  1. | 2010          e         1        1 |
  2. | 2010          a         1        1 |
  3. | 2010          b         1        1 |
     |------------------------------------|
  4. | 2010          c         2        0 |
     |------------------------------------|
  5. | 2010          d         3        0 |
     |------------------------------------|
  6. | 2011          e         1        0 |
  7. | 2011          b         1        0 |
     |------------------------------------|
  8. | 2011          c         2        1 |
  9. | 2011          a         2        1 |
     |------------------------------------|
 10. | 2011          d         3        0 |
     +------------------------------------+

For more discussion, see this FAQ and this tutorial review.