The season 7 "Roleplaying Guild Guide" (the new name for the Guide to Organized Play), available for free from Paizo, discusses what to do in this scenario on page 33. I hope this helps.
Determining Subtiers
In order to determine which subtier a mixed-level group
of PCs must play in, calculate the group’s average party
level (APL). Divide the total number of character levels by
the number of characters in the party. You should always
round to the nearest whole number. If you are exactly at
0.5, let the group decide which subtier they wish to play.
Starting with Season 4, scenarios are designed for six
characters and contain instructions on how to adjust the
scenario for four-character parties. When the APL of a table
is between two subtiers (like APL 3 for a Tier 1–5 scenario),
a party of four characters must play the lower tier without
any adjustments for party size. A party of five to seven
characters whose APL is between two subtiers must play
the higher tier with the four-character adjustment.
For scenarios written in Seasons 0 to 3, when the APL is
in between subtiers, a party of six or seven characters must
play the higher subtier. Parties with four or five characters
must play the lower subtier. In the fringe case where there
are no players that are high enough to have reached the
subtier level (such as a party of six 3rd level characters),
the group may decide to play down to the lower subtier.
Legal Table Size
The minimum table size for a Pathfinder Society
Organized Play session to be considered legal is four
PCs. Table size is capped at six PCs. In cases in which
you simply cannot seat four players, you may run a table
of three players, and play an official level-appropriate
pregenerated character in order to meet the minimum
table size of four PCs.
Conversely, if seven players show up to an event,
rather than turning someone away from the campaign
altogether, consider adding a seventh person to the table.
These situations should be extremely rare and should only
be used as a last resort to sending someone home without
the chance to play. Seven-person tables often overpower
otherwise challenging adventures and limit the amount
of time each player gets to shine in the given scenario, and
many players prefer not to play at all rather than play at
such a large table. Check with the players to determine
their preferences before running a seven-person table.
One alternative to a seven-person table is to split the
group into two tables of three players, and ask one of the
players to serve as GM for the second table, with each GM
running a pregenerated character to fill her table out.
Tables should never have eight or more players.
