If you want to stop members leaving a group there are two options:
- In the group go to the Settings section, choose the Advanced settings tab, then Resource permissions and then the Member resource type. For the Owner role remove the Delete operation and select the Save button. Now only group administrators or moderators will be able to delete members.
- In the group go to the Settings section, choose the Advanced settings tab, then Member workflow. Change the Approve leaving setting to Yes and select the Save button. Now administrators will be given a new task of approving or rejecting when a member wishes to leave the group.