How does T-Mobile's add-a-line work?

A family member has a family plan that they pay around 50 or so /month and thought about adding me to the plan.

•How much is the added plan if I chose the unlimited talk & text unit?
•Is there one that is only 500 anytime minutes & unlimited text?
•Does that one come with unlimited nights & weekends?
•How exactly does the unlimited nights and weekends work? (I previously read that it’s unlimited talk to any landline & unlimited text to any phone. Landline as in ‘no cell phones’?
•Additional information is highly appreciated.