Learn Turkish: Projedeki bütün paydaşlar toplantıya katıldı. - All stakeholders in the project attended the meeting.

The root is pay, which means "share" or "portion." The suffix -daş (which can also be -taş or -deş) creates nouns indicating fellowship, sharing, or a common bond. So, a paydaş is literally "one who shares a portion"—in business or projects, this translates perfectly to "stakeholder." The -lar suffix simply makes it plural.

Why is it projedeki instead of using a possessive like projenin (the project's)?

Projedeki uses the locative case -de (in) followed by the relative suffix -ki (the one which is). It literally means "the ones who are in the project." You absolutely could say projenin paydaşları (the project's stakeholders) using a genitive-possessive construction, but projedeki emphasizes their active placement or involvement within the project itself.

In English we say "attended the meeting" as a direct object. Why does toplantıya take the dative suffix -a?

The verb katılmak is usually translated as "to attend," but it more accurately means "to join" or "to participate." In Turkish, you always participate to or join to an event. Because of this, katılmak always dictates that its object takes the dative case (-a/-e). Therefore, toplantı (meeting) takes the buffer y and the dative -a to become toplantıya.