Learn Finnish: Hän on ryhmän kokenein insinööri. - He is the most experienced engineer in the group.

It comes from the verb kokea (to experience). First, we form the past active participle kokenut (experienced), which has the inflectional stem kokenee-. To make it superlative, drop the final vowel of the stem and add the superlative marker -in, resulting in kokenein.

Why is ryhmän in the genitive case instead of using a location case like ryhmässä to mean 'in the group'?

In Finnish, it is highly idiomatic to use the genitive case when expressing a superlative out of a specific group, literally translating to 'the group's most experienced'. While using the inessive case ryhmässä wouldn't be strictly wrong, the genitive is much more natural and common in this context.

If we changed the sentence to say 'I spoke with the most experienced engineer', how would kokenein inflect?

The graded participle acts just like a regular adjective and must agree in case with the noun. Words ending in the superlative -in use the stem -impa- or -imma- depending on consonant gradation. For the adessive case, it becomes kokeneimmalla insinöörillä.

Are superlatives like this only formed from past participles?