Google, Microsoft Office 365 and various other programs online have a collaboration feature that allows students to work together on the same document. This is pretty cool and can be invaluable for developing essay writing skills in a social constructionist point of view.
This is a great activity, I like to pair strong and weaker students together as it allows for mutual learning - Pedagogically I am thinking of Lev Vygotsky's 'Zone of Proximal Development.'
Before carrying out the activity, you must have the students in groups plan out the essay into paragraphs including notes about what they plan to say. Before setting the task, have each group member pick a paragraph to work on. You must also consider a grading rubric that can be given to the class afterwards to enable them to grade the work.
1. Set up a document on Office 365 or google drive. Share the document with a small group of students, and give each student editing access in the share feature.
2. Give the class a timed period to finish the essay.
3. Have the group then grade the essay as a whole document. Get the group to identify three weaknesses and three strengths.
4. Finish the activity by asking the students to write a reflection about what they have learned about essay writing through the procedure. I normally set this as a homework activity.