First, there are only 2 ways to join an IC to a unit before the start of the first turn. Either by deploying the IC within 2" of a unit, or by declaring that they are joined after they have both been placed into reserves.
Now that that's established, the order is very important:
1a) your deployment
1b) opponent's deployment
2a) one player deploys infiltrators
2b) other player deploys infiltrators
During step 1, all units and ICs must be placed on the table or into reserves. Then, in step 2 you make the same choice for infiltrators. Given this, all ICs and units without infiltrate will already be on the board or in reserves by your deploy infiltrators step. So, being deployed as an infiltrator can never be conferred either way.
If the IC and unit are joined in reserve, they may benefit from outflank as conferred by infiltrate.
The rule that @scscofield
mentioned is a very odd addition. When placing models, it translates to, "Infiltrators may never deploy within 2 inches of an IC without infiltrate." This prevents farseer/rangers and ethereal/kroot even fully within your deployment zone.
The part I am unsure about is whether or not this rule prevents an IC without infiltrate from joining a unit with infiltrate in reserves as this is technically during deployment. I'm inclined to say this is true. So, the only thing you can do is use an IC with infiltrate to outflank a unit without.