Sign into Xanga, and visit the blogring you'd like to stop moderating.  If you're the leader, at the bottom of the left module you'll see some links that say:

Settings:
» Profile
» Approval

Go ahead and click on "Approval".  There should be a setting labelled as:

Members:  Automatically approve new members 

If you're turning auto-approval back on, than the setting is probably set to "No".  If you change the toggle to "No" and click on "Save Changes", members will once again be automatically approved.

PLEASE NOTE: If there are any pending applications when you turn auto-approval back on, they will be automatically approved!