Design advice regarding a users/groups/tasks database scheme
Try these design which will satisfy your requirements.

Group

 - Group Id Primary Key
 - Group Name)

User

- User Id Primary Key
- Name
- Email 
- Created on 
- Group Id references Group(Group Id), Referred User ID references User(User ID))

Task

- Task ID Primary Key 
- Name 
- Priority 
- Date)

User-Tasks

- Relation ID Primary Key 
- User ID references User(User ID)
- Task ID references Task(Task ID))