I wish I hadn't read this thread, because it has started to confuse even me
I think what is confusing people is the use of 'BP' for build points and base points.
So - Base Points = the cost of a character/agent/civilian
Build Points are used to buy weapons/gear/ special agents.
Skills can only be purchased by winning a scenario, which adds to BASE POINTS
For the FIRST GAME you can only have a total of 15 BASE and BUILD points.
AFTER the first game, you can spend BUILD points to your hears desire - which could be bad news if the lurker player has a lot of points to spend.
therefore - you can still add models to a Threshold list provided you have not spent 15 or more BASE POINTS.
If a model dies and the removal of that model's BASE POINTS from the list gives you extra BASE POINTS to spare (i.e. the value of BASE POINTS drops below 15) you can add new models to the list, otherwise you can not add another model.
sorry for typing that out - I just needed to get it (back) into my head after reading this post