On our 2.6.0.beta2 forum normal users (TL3) are unable to add someone to a PM topic. The Add or remove button is there, they can remove with the X, but the + button is missing. For admins and moderators it is there and works fine.
From other topics here on meta I understand that this should work for normal users as well. I cannot find a setting that seems to influence this.
I don’t know if it worked before. We’re online for 3 years and I just noticed, never had the impulse to add someone before and nobody ever complained.