Learn Turkish: Teşkilata katılıp casus oldu. - He joined the agency and became a spy.

In Turkish, the verb katılmak (to join / to participate) always requires the dative case for the thing being joined. You can think of it as joining 'to' the agency, which gives us teşkilata.

What is the -ıp suffix doing at the end of katılıp?

The suffix -(y)ıp connects two actions done by the same subject, acting like the word 'and'. It attaches to the verb root (here, katıl-) and borrows the tense and person from the final verb in the sentence (oldu). So, katılıp... oldu means 'joined and... became'.

Could I just say Teşkilata katıldı ve casus oldu instead of using -ıp?

Yes, grammatically that is completely fine and means the exact same thing. However, using -(y)ıp to chain verbs together is much more common and sounds more natural in everyday spoken and written Turkish than using the word ve.