I know this is quite an old post, but it is still relevant. 

The first thing to do is to sit down with the Lead Developer, the Product Owner and yourself (Plus the Project Manager if there is one). Go through the basic Features or Epics of the project, and write them down. Put these on sticky notes so you can move them around. Sort them out into Epics and Features first of all. 

You haven't said what Agile methodology you are working to. Sadly despite what people say, there are lots of variations. I am assuming you are working with Epics and Features and User Stories.

Having sorted out the Epics, then place the Features underneath each Epic. Make sure this is agreed by everyone. This is your basic framework. 

In time, you as the BA will start writing the User Stories but don't worry about them right now. 

Ask those present what order they should be worked on. The Lead Developer should have the biggest say on this, Sort out the Epic/Features in order by numbering the Sticky notes - 1 = the one to be worked on first and so on. 

You should now have an order of work. 

Next, as the BA, you now need to write the User Stories as usual. Depending on how you work, you need to ensure that you are writing as many User Stories as the Developers are working on. Do you as the BA do Show and Tell's or does the Project Manager? If it's you then make sure you arrange and hold all the daily stuff - stand ups, requirement gathering workshops etc. Are you using Refinement session and Sprint planning - this will also need to be factored in and can act as another way of Developers agreeing what to bring in and when. 

This should give you a development plan. The Product Owner and PM might have a say on the order depending on expectations from the client. 

The PO and PM will want regular updates but they should be attending the daily Stand-ups so should know this from them. You may though, want to hold a weekly/monthly meeting to catch up with each other.