First submit a poem as a blog post in the workshop category for the current month and year. Any member can then add a comment, which when approved will appear below the poem. There is no limit on the number of comments that can be made. The author of the poem can respond to any of the comments.
Have a look at the WordPress documentation about posting messages here. You should remember to choose the correct category for your post. Don’t be scared of using the site – anything you do has to be approved before it’s made public. The only way to learn is by jumping in with both feet and learning to swim.
When you have published your entry you can click on the link at the top right of the Edit Post page where it says Notify Users about this Post. This will bring up a page where you can send an email to everyone. The easiest way to specify the recipients is to click on administrator and author rather than select all users individually (unless of course you only want to send it to a few people).