For that you need select the option "Read Only, with Local Groups" in LDAP Permissions section and then appears the field "Default Group Memberships" where you can put the default group. More info: https://confluence.atlassian.com/display/JIRA/Connecting+to+an+LDAP+Directory
I recommend you to take a look on the following articles from our documentation, which may be very useful:
-- Arthur Gonçalves
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