Learn Finnish: Me keilattiin ja sen jälkeen me mentiin baariin. - We bowled and after that we went to the bar.

In spoken Finnish, the passive form is used almost exclusively for the "we" form. Here, keilattiin is the past passive of keilata (to bowl). While standard written Finnish uses me keilasimme, everyday speech uses me keilattiin.

Doesn't the passive usually mean the doer of the action is unknown?

Yes, in formal written Finnish! But in spoken Finnish, adding the pronoun me (we) before a passive verb gives it a specific doer. Me mentiin clearly means "we went", completely replacing the standard written form me menimme.

Why does keilattiin have a double "tt" while mentiin only has one "t"?

This happens because they belong to different verb groups. Keilata is a type 4 verb, which takes the -ttiin ending in the past passive. Mennä is a type 3 verb, which takes the -tiin ending with a single "t".

Why is the word se in the genitive form sen before jälkeen?

The word jälkeen is a postposition meaning "after". Postpositions almost always require the word that comes right before them to be in the genitive case. So se (it) becomes sen to form sen jälkeen (after it or after that).