Adding an existing confluence User isn't working. You receive no errors in the UI. You can see an exception inside the LogFile.

2014-06-18 13:40:40,766 ERROR [http-bio-127.0.0.1-8090-exec-14] [common.error.jersey.ThrowableExceptionMapper] toResponse Uncaught exception thrown by REST service: userkey
– url: /rest/prototype/1/search/user.json | userName: christians | referer: https://wiki.innoq.com/calendar/mycalendar.action
java.lang.NullPointerException: userkey
at com.google.common.base.Preconditions.checkNotNull(Preconditions.java:204)
at com.atlassian.sal.api.user.UserKey.<init>(UserKey.java:27)
at com.atlassian.confluence.plugins.rest.entities.builders.PersonalInformationContentEntityBuilder.build(PersonalInformationContentEntityBuilder.java:123)
at com.atlassian.confluence.plugins.rest.service.DefaultRestSearchService.toSearchResultEntity(DefaultRestSearchService.java:274)
at com.atlassian.confluence.plugins.rest.service.DefaultRestSearchService.toSearchResultEntityList(DefaultRestSearchService.java:263)
at com.atlassian.confluence.plugins.rest.service.DefaultRestSearchService.makeSearchResultsEntityList(DefaultRestSearchService.java:223)
at com.atlassian.confluence.plugins.rest.service.DefaultRestSearchService.userSearch(DefaultRestSearchService.java:76)
at com.atlassian.confluence.plugins.rest.resources.SearchService.doUserSearch(SearchService.java:194)
at sun.reflect.GeneratedMethodAccessor3966.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
…
at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
at java.lang.Thread.run(Unknown Source)
2014-06-18 13:42:03,791 WARN [http-bio-127.0.0.1

I was searching for the user "rj" (Rainer Jaspert) (rj is his username)