I do not believe you should make a secondary forum, but you should sequester all creative efforts to "members only".
Works (stories, poems, artwork, plays, etc ...) that are visible to the outside world are considered
published.
Any publisher down the line would be picking up secondary rights for something already published.
Or they may not even consider your work at all.
You could build a chamber at TLO where members could share their works with fellow members.
Not all may place efforts in there. For example, me, I couldn't are less about my perky film reviews.
They are on my website, as well as open forums and closed forums.
My essays, however, those are different and are on a subscription.
This is much discussed at various sites, but a short answer =
Is a literary work posted on a story sharing site considered published work?
archive.avvo.com
I welcome all works here, but I would not want our open door policy to potentially harm aspiring or fledgling artists.