Here’s what I found:
- TL2+ users are able to add other users to PMs whether they are the topic starter or the recipient
- TL1 PM creators cannot add or remove users.
- TL1 PM recipients can remove themselves, but not add any new users.
According to our Trust Level breakdown, the above behavior seems correct (emphasis added):
@laughingriver the best thing to do would be to get the site updated. After that we can try to figure out any other factors that might be coming into play.