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I misremembering something, but is there a rule that says a units special rule or ability only functions if the unit is on the table at the start of the turn? For example, The Pinion Company allows scout sergeants to grant ignores cover. They also can lead units in an outflanking move. If the scouts and the attached unit outflank, can the sergeant give his Ignores cover ability to the attached unit? I saw an argument that says special rules like that requires the unit being on the table already at the start of the turn. Thanks!

It is dependent on the individual rules.

This one in particular says nothing about being on table at the start of your turn, and in fact isn't even used until your shooting phase. So, yes, you may use it the turn you outflank in.

What Kel said.

For example the Space Marine locator beacon states, "The locator beacon must have been on the battlefield at the start of the turn in order for it to be used."

I believe he's referring to this tidbit from 'Moving On From Reserve' (pg. 136, main rulebook):

A unit cannot charge, or use any abilities or special rules that must be used at the start of the turn, in the turn it arrives from Reserve.

GW seems to ignore this rule willy-nilly, so its hard to say when it applies at times.