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Without any other context, I would translate it as "They (?) said it was also possible to join the club for something called a trial period or something".
"Taiken Nyuubu" means joining a club just to try it out, such as when school first begins and the new students aren't sure what club to join.
There is no real "reason" why "ari" is in katakana, as far as I know.
Hope this helps
* アリ could be 有り in kanji, but it looks more formal.
* アリ could be あり in hiragana, but (1) hiragana letters are too difficult to read when you have too many; and (2) they look childish. (Childish <---- Children can't write Kanji well.)
* アリ in Katakana can replace kanji. A slang, a joke and a colloquial word could also be expressed in Katakana.
There may be more reasons.
...... もアリかと。 ＝ You could ....
Last edited by mikkih; May 04, 2011 at 12:00 PM. Reason: added the last