APL is Average Party Level, right? If the players are all the same level, wouldn't the APL (and therefore ship tier) be the same regardless of whether there's 3 of them or 9 of them?
Well, you don't need to worry about followers/companions, just the PCs themselves. Unless you have a great disparity in their levels (which creates its own problems for adventuring), just go with the overall average of regular and semi-regular PCs and gradually move up the ship tier as the overall group slowly levels up.
If your PCs are different levels, then just average all the PCs in the whole group, whether the players show up or not.
That way, it can simulator the camera just focusing on a few PCs at a time, while all the other PCs are just in the background doing something else.
Also, just to note, since I've seen a couple people make what I am 99% sure is the same mistake:
BP don't go up with Tier. Tier goes up with BP. Or to be more specific, your ship Tier doesn't go up *until you actually spend the BP to improve it*. Your ship doesn't magically get harder to pilot, and start facing tougher foes, until after you *actually get the improvements for improving your APL installed*.
This is the common sense way to do it, but unless they've made an Errata, it is not what the rules state. The rules state that your Tier is equal to the APL, regardless of how many build points you've used.
The rules are funky in this regard.
Also, just to note, since I've seen a couple people make what I am 99% sure is the same mistake:
BP don't go up with Tier. Tier goes up with BP. Or to be more specific, your ship Tier doesn't go up *until you actually spend the BP to improve it*. Your ship doesn't magically get harder to pilot, and start facing tougher foes, until after you *actually get the improvements for improving your APL installed*.
This is the common sense way to do it, but unless they've made an Errata, it is not what the rules state. The rules state that your Tier is equal to the APL, regardless of how many build points you've used.
The rules are funky in this regard.
If we're really going by RAW, then tier equals APL when APL increases. There are no rules for decreasing a ship's tier. It never actually says your tier is always equal to your APL. If APL goes down for some reason (characters die or leave), the tier doesn't change.
As the PCs go on adventures and gain experience, they need
an increasingly powerful starship to face tougher challenges.
When the characters’ Average Party Level increases, so does
the tier of their starship (see Table 9–1: Starship Base Statistics
on page 294). The PCs receive a number of Build Points equal
to the Build Points listed for their starship’s new tier – those
listed for its previous tier, which they can use to upgrade their
starship.
I'll note their is a "can use to upgrade" as opposed to must upgrade.
Also, if all ships are automatically the tier of the party, the multiple ship rules on page 326 don't make any sense.
Multiple Ships
If the PCs have more than one starship, use the highest-tier ship’s
tier as a base and add 1 to this value for each additional starship
within at least 2 tiers of that starship. If none are within 2 tiers,
add up the tiers of all the additional starships and add 1 to the base
value if the total is equals or exceeds the base starship’s tier. Use
this modified value when determining the encounter’s difficulty.
Why have rules for PC ships which have different tiers, if the intention was for every PC ship to have tier = APL. While RAW may be unfortunately worded, RAI it is clear that PCs could step onto a ship and not change its tier instantly. And in fact, can choose to have ships at tiers less than their APL, per the quoted multi-ship rules above.
The party ship is the same level as the party average -- but it is certainly possible to have NPC allied ships or extra ships borrowed by the PCs.