Establish roles for each member contributing to the proposal writing procedure. Divide roles according to stages of the writing process (brainstorming, first draft, editing and revising, finalizing, layout, and so on. See Resources) or for different components of the proposal itself (abstract, introduction, executive summary, and so on). Establishing roles immediately will ensure that each group member contributes to a comparable degree.
Set a working schedule for the different stages of the proposal writing process. Again, the stages can follow the writing process itself, or they can focus on the elements of the proposal. Use deadlines and workshop days to ensure that even if members work independently, everyone is still able to come together and share their progress. This will ensure cohesion between group members which will in turn lead to a more cohesive, stable final proposal.
Meet regularly to read and review each member's contributions to the writing process. Early meetings should focus on early components of the proposal (letter of transmittal, abstract, executive summary, problem statement), while later meetings should relate specifically to the research elements of the proposal (literature review, technical plan, budget). Your group's final meetings should focus on "extraneous" components of the proposal such as table of contents, graphs and other graphics and appendices, as well as the overall layout, format and presentability of the final draft.
Swap and share different sections of the proposal, both individually throughout the writing process, and again as a cohesive proposal at the end of the writing process. Edit and revise each section both individually and as a cohesive whole. This will help to ensure a unity to the proposal that could evaporate given the group collaborative effort.