Another useful resource (co-authored by Rowena Murray) is Sarah Moore's 'Handbook of Academic Writing' I attended a workshop she hosted last year and found it really useful. In her book, she discusses three underpinning principles of participating in writers groups:
1. Writing is a social act that benefits from discussion with peers.
2. Feedback can be helpful at different stages in the writing process.
3. Working in a group can motivate writers to initiate and progress projects and produce successful outputs.
[Moore, S. (2006) Handbook of Academic Writing. Berkshire: Open University Press. p.111]