People and plans
Everyone on your account is covered by one plan price. There is no per-person charge and no limit on how many people you add, so the questions worth answering here are who can do what, and what actually bounds your spend.
What actually limits your spend
Nothing about your team size does. Your plan includes an amount of AI spend each month and bills a small amount for each block above it, whether that spend comes from one developer or twenty. The only thing that stops work is the monthly spending limit you set yourself, from the subscription page - below it you are billed for what you use, and at it we stop charging.
That is also why per-developer budgets exist. A single account-wide limit is the right default, and a per-member budget is how you cap one person without capping everyone.
Adding and removing people
Invite anyone from the Team page and they join as soon as they accept. Nothing is provisioned first and nothing is charged for the person: adding the fifth or the fiftieth colleague does not change your bill. Removing someone revokes their access immediately.
Converting to a business account
An individual account can become a business account by adding your company details, which is what puts a company name and address on the invoice rather than a personal one. Converting is free and does not change your plan.
Do this before you invite anyone, not after. It is the difference between an invoice your finance team accepts and one they send back.
The roll-up
A manager view of the team week by week: who moved, what changed, and who is barely using Lineman at all. That last group is worth knowing about - not because they cost you anything, but because they are the people not yet getting the savings.