Which tool should your group use? That depends on what aspect you want to foreground in your narrative--chronology or geography? If your research leads you to want to emphasize chronology--change or continuity over time, the role of contingency in history--then Timeline JS is probably the better choice. If you want to emphasize the importance of place (including questions of distance), then StoryMap JS might work best.
You do need a Google account to use this tool. While you can set up a shared gmail account so that everyone can access the project, please note that some groups have found that work done simultaneously by more than one person is not saved. Also, be aware that one person's mistake can eliminate an entire project (and yes, it's happened, unfortunately).
Each slide within your StoryMap project should include a caption of 25-150 words. These captions are interpretive acts! You are not just conveying information; you are attempting to provoke a response in your audience. When you have only a few words in which to do that, every word counts!
Beverly Serrell, in her Exhibit Labels: An Interpretive Approach, 2nd edition (Rowan & Littlefield, 2015) suggests that you focus on trying to answer the following questions in your captions:
The Getty Museum's Guide to Adult Audience Interpretive Materials suggests a very similar set of questions: