Sometimes you will want to collaborate with others on a topic in the forum - particularly when drafting a document, coming up with ideas, etc. For this you can use the 'Wiki' functionality of the forum:
1. Create a new Topic as normal.
2. Write your post, and click 'Create Topic'
3. Once you've posted it, you can make your post a wiki by clicking on the ... [show more] button:
4. select 'Make Wiki'.
You made a Wiki!
Now anybody can edit a wiki post by clicking on the pencil button  below the original post
(NOT the pencil at the top right, that just shows the edit history of the post).
It is a good idea for anybody who edits the wiki to mention what they added/removed and why.
NOTE: You can only make wikis from topics that you created. Discourse (our forum software) sets 'trust levels' for users depending on how much they have interacted on the forums, how many posts they have, how long they have had their account etc.
By default, 'Make Wiki' isn't available for new users, so if you need to make your post into a wiki and the ... [see more] button isn't visible for you, just send me a direct message, and I can change your trust level.