Yes. First, you must join the membership.
Because the membership has a free trial, there is no cost for joining. When you join, an account will be created with Lenspiration. This account is what allows you to participate on the forums.
If you cancel your membership, you will lose access to all membership features, but since you will still have an account with Lenspiration, you will still be able to participate on the forums.
Regardless of how long your membership lasts, you will have access to the forums through your account even after canceling your membership. Therefore, if you want to participate in the forums for free, join the membership and then cancel your membership before the free trial expires.