In Service Desk 2.0 and above with public signup, Customers are added into the certain groups and count towards JIRA license. As a result, Service Desk Customers are able to login JIRA and perform actions based on the permission given to the configured groups.
By default they should not be added into any groups.
JIRA is configured to external user management such as LDAP or Crowd.
This is an expected behaviour as Default Group Membership option is enabled for JIRA to LDAP or Crowd, thus new users will be automatically added into the configured group(s), usually jira-users so they can login.
Disable Default Group Membership in LDAP or Crowd.