There are a number of Moodle activities that can be used to do what you want. You could use a regular assignment set up for online text. You could use a database activity with a field for the text, and make sure you put the edit icon on the individual entry template.
Also, as you mention, you could use the Journal activity. To get the complete functionality you mention, you will need to create a new role based off the student role with the added capability to mange journal entries. Create your different groups. make sure each group includes the facilitator for that group. Set the Journal to use visible groups.
Send me an email if you would like access to an example journal set up for this. drachels at drachels dot com.