At this stage, a personal study cannot be switched to a group study where you can invite members. The best way to invite people to study with you is by going back to the course library, selecting the course again, and choosing to study “with friends” when prompted. You will then be taken to additional steps where you can invite your member(s) via email.
This article takes you through how to Start a Group/Course.
You also can always add more members to your group by following the instructions in this article: Add/Invite Group Members