"Ĉiu individuo partoprenas ian grupon."

Translation: Every individual participates in some kind of group.

[JanPaulido]

Would it be proper or acceptable to say " partoprenas EN ian grupon " ?

[mizinamo]

Nearly. You can use en but then with nominative: partoprenas en ia grupo.

(Because you "take part in the group" and not "take part into the group".)

[JanPaulido]

Thank you. That helps :)

[Kvasxaro]

Shouldn't it be "iun" unstead of "ian"?

[mizinamo]

ian grupon - some kind of group, some type of group, some sort of group.

iun grupon - some group.