You as the overall project group owner must control and set up the procedures as Zhang stated. Most importiant is to get the contractors off to a good start. This means you will
control activity codes
control calanders
control ties between contractors
control update managment
control autocost structures.

This is sooooo critical, as if one contractor uses a code field and so does another use the same field, values can overwrite each other when you inport export.
This invites a disaster.

Best way to do this is to call all contreactor planners in for a meeting, outline how you plan to manage the schedule,codes, etc.

If this is not possible, then you must set up the schedule group, assign ids for each contractor and make projects within the group for each contractor.
In doing this, you will set up assigned code structures, as you have limited codes 20,and 64 slots. and values and then you will set autocost rules and scheule rules.
After this is done, you will send each contractor his basic schedule, already set up, with no activitis. he will populate the schedule accordingly. and return to you to iput into the group.
again, you must control codes and ids.
best way is to assign code fields to your self and to each contractor.
example, managment will use the first 10 fields.
contractor a will use code field 11 and 12 only for his use but his code values must start with x, next contractor will use same fields but his activity codes must start with r

and so on and so on. all can use your managment codes but only you can change them.

same with all other data, you control
then you will find it to be easy to update your schedules and manage the job.
good luck
cheers bill