The topic should echo the nature of the journal -- to reflect your close relationships or to get to know each other better. If the purpose is to celebrate a group of intimate friends, you can be more open-ended with the topic. Your crew of chums has an existing bank of shared memories to draw from.
This circle journal included a template for each participant to fill out that included name, age, and location. If the objective is to get to know people better, picking a specific topic may be a better option.