Postlab pricing is per team member. Adding members is super simple: you can add members by clicking the big green + button that's all over the app, and also available in the Preferences. There, you can also remove them at will.

When adding a member, you will be charged for that member for either a month or year - depending on the billing cycle you're plan has. Removing a member can be done at any time, and any unused days within that billing cycle are credited to your account.

Generally, I will only collaborate with one other person, but they may change from time to time so I’m not sure how to best manage that?

Just add that person, and when you're done, remove them. We'll credit you for that user's non-used days.

I already have an account. Should I be invited as an external member or as a team member?

Other teams can indeed invite you as an external member, but that will only get you access to the Productions - not to their Drive. If you plan on using Drive to share media, all members need to be on the same team. If not, inviting you as an external member suffices.