Would it be proper or acceptable to say " partoprenas EN ian grupon " ?
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".)
Thank you. That helps :)
Shouldn't it be "iun" unstead of "ian"?
ian grupon - some kind of group, some type of group, some sort of group.
iun grupon - some group.