the route

A(0)---x2------x1----x3------x4----x6------x5----B(x7=1000)
|---t1-------->|~~~~~|~~~~~~~|~~~~~|~~~~~~~|~~~~~|
~~~~~~~|<--t2--|~~~~~|~~~~~~~|~~~~~|~~~~~~~|~~~~~|
~~~~~~~|--------t3-->|--t4-->|--------t5-->|~~~~~|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~|<--t6--|~~~~~|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~|--------t7-->|

1. take #2,#3 to x1
2. back, pick up #5 and meet #4 at x2
3. meet #2, change #2 and #5 at x3
4. meet #3, change #3 and #4 at x4
5. leave #2,#3 at x5
6. back and meet #4,#5 at x6
7. all persons arrive B at same time

Note: if you repeat from t3 to t6 for N times, you can get better result.