As the Topic says.

1. We are using the trigger only feature.
2. All members of the xbmc foundation are whitelisted for triggeronly
3. When a member now triggers by commenting the triggerphrase on a PR it only works when the PR was done by a whitelisted/team member

This line of code is the problem:

https://github.com/jenkinsci/ghprb-plugin/blob/master/src/main/java/org/jenkinsci/plugins/ghprb/GhprbPullRequest.java#L290

It checks the "accepted" flag which is only true if the author of the PR was whitelisted. It is set to true here:

https://github.com/jenkinsci/ghprb-plugin/blob/master/src/main/java/org/jenkinsci/plugins/ghprb/GhprbPullRequest.java#L77

IMO the "accepted" flag shouldn't be checked at all in the triggeronly scenario. Its enough that the commenter is whitelisted.