how do/can you get trainers in your secret base?

[Zallarion]

i saw it at a trailer

[Jeff]

If you mix records with other people then their secret bases will appear in your game. If you go in someone else's secret base you will see them in there and will even be able to fight them, and they will have the same pokemon they had when you mixed records.

[mr_pikachu]

Quite right. And by the way, you will be fighting the trainer as if he/she was a regular in-game trainer. That is, you can use items and such, and you will get experience and EV points for winning (as well as money, I believe). However, you can only fight each Secret Base trainer once per day, unless you mix records with that person again before the day ends.