This "problem" comes from the fact that LDAP GroupOfNames (or GroupOfUniqueNames) requires the member (or uniqueMember) attribute to have a value.
This issue is commonly regarded as a undesired restriction as it doesn't allow creating/maintaining empty entries of these types.
Most common solution or better workaround this is by providing a "dummy" or known value always, or by always using the entry own dn as default value.
The "uid=foobar" is just an example "default" required value configured in the security-ldap.xml assembly configuration, which you can change to something else if you want.
However, I'm going to provide an alternative solution which will allow you to use a predefined marker value, "#dn", which Jetspeed then will replace automatically with the current entry its full qualified dn itself.