Install the attached unofficial fix cq-6.3.0-patch-CQ-4215472-1.0.zip
Classic UI User Admin tool unexpectedly removes group members when adding more
Classic UI User Admin tool unexpectedly removes group members when adding more | AEM 6.3
In the Classic UI User Admin tool , when adding users to a group that has more than 35 members, members are removed as the new ones are added.
Known AEM UI product bug CQ-4215472
The issue is fixed in AEM 126.96.36.199 (SP1+CFP2) and later cumulative fix packs.
There are two possible workarounds until then:
Or use the Touch UI instead http://host:port/libs/granite/security/content/groupadmin.html