Moderators can set Join Questions which users will be required to answer before joining a group. They can can serve as a screening process for Moderators.
Groups need to have Restricted Accessibility in order to set Join Questions. Learn more about Group Accessibility here. To set Join Questions:
Click on "Group Settings" in the right side panel.
In "Settings", scroll down to "Access".
Click on "Restricted".
Toggle on the button "Require people to answer questions when requesting to join this group".
Set your Join Questions.
Note: Join Questions do not show up for users invited to the group through Hylo. They are only accessible if a user organically finds the group OR if they click on the unique join link.
Read more about inviting members here.