You need two toons to be in the group, whose accounts are directly RAF linked.
The toons need to be 79th or lower, and within +/- four levels of each other.
They need to be within a certain proximity of each other.
And the quest completed, or mobs killed, cannot be grey to either of them.

If your chain is A > B > C > D > E > F
That is six accounts.
You cannot have more than five in a party at once.

If you want A to get RAF, B needs to be in the party; if you want F to get RAF, E needs to be in the party.
Aside from those restrictions, pick four accounts ABCD or BCDE or CDEF, and go with a booster (linked or not).

If your RAF is new, call billing to change your chain:
A > B > C
A > D > E
With a booster on A, and four linked accounts.
BC and DE are both valid RAF chains, and A gets a lot of granted levels so doesn't lose out from always using a booster character.