To add someone to your account you send them an invitation from your profile. Only the owner and administrators can do it (see «Your team: who can do what»).

You type their email address and choose the role they come in with, administrator or member. They get an email with a personal link that expires after 7 days and works only once. When they accept it, their user is created inside your account.

Nobody can be invited straight in as owner: that role belongs to whoever created the account.

Your plan sets how many people can be on the account, you included (see «Plans and advertising spend limits»), and an extra user adds one seat. An invitation that has not been accepted yet holds its seat until it expires after 7 days, so with the plan full you cannot invite anyone else: revoke a pending invitation, remove someone, or upgrade your plan. Resending an invitation that has already expired gives it its seat back, so with the plan full you cannot do that either. Accepting an access request counts the same way, and if there is no seat left the request stays pending. If you move to a smaller plan nobody is removed from the account; you just cannot invite more people until there is room.

If someone from your company registers on their own with an email on the same domain as yours, instead of creating a separate account you get a request to accept or decline.