SparkChat Public and Private Groups are used for the following purposes Public Groups are open to all members in your team/ Company and anyone can join or leave the chatroom whenever they wish to.Private Groups are used when you want to speak to a specific set of people about a certain topic. Group Members can be added to a group only by the admin (generally the team member that has created the group) and is by invite only.
This will depend on the role assigned to and the features made available by the admin of SparkChat in your team/ company. This feature may or may not be available to you basis the feature set made available by the admin.