There are two ways for a user to request a membership, and both have different ways to enable auto-approving a membership request.
Members can join your community in two ways:
Membership invite by link
Community sign-up page
How to auto-approve membership requests when a user joins through membership invites:
When creating an invitation by email or by link, you can untick or tick this checkbox to enable auto-approval of membership request
Click this link for more information about membership invites or watch this video
1. Click the Admin Panel button.
2. Click Settings on the side menu.
3. Tick the Automatically Approve Members Checkbox to enable the auto-approved membership request when a user joins the community through the community sign-up page.