Yes. Discussions are limited to group members & they are all free!
If you posted in a discussion in process look at the top of the discussion that you are posting in and see if it shows page 1,2 etc. If the discussion has been replied to a lot of times, it will go into a 2nd page and your post could be showing up there. A signature is the small message you see under a member's post.
Click [settings] next to My Profile in your left personal menu, and then on the forum settings there you will see signature, type your tagline in there and click update. You may start wherever you wish, but there is a introduction part of the forums on the group, where most will start, by introducing themselves and telling a little bit about themselves. You can also check out the ‘new to the group?' link in the group's main page. Click on the groups forums menu item in the group menu (on the left). Click on the appropriate category and then click on the start a new discussion button.
While reading a discussion you can click on the start a new discussion button to start a new a new discussion in the same forum category. When logged in you will find an edit button at the bottom of your post. A lock icon means that no new posts are allowed.
Few possible reasons:

• It is used by the group leaders or site managers for special messages and information purposes.
• Group leader or site manager has decided to lock it (discussion got off topic or too "hot").

The thumbtack next to a post is for keeping that post near the top of the forum, it is used for showing that this is considered an important or informative topic. In the post editor above the text area you have a drop down menu to change the color and size of the fonts. You can also click the help? next to it, to learn of more options.