Can a group have more than one study going at a time?
Your group is limited to one study at a time. You can create as many studies as you like, but your group will have to finish one study before you introduce a new study.
When you've finished a study and are ready to start another one, click on the pencil icon. Under the "Study" tab, select which study you would like to add to your group.
Scroll to the bottom of the page and click "Save Group Study." That will automatically remove the study you just finished and add the new study to the group. No need to re-invite your group members to the study. They'll have immediate access.
There is no limit to the number of studies you can create. Just remember a group can only have access to one study at a time.