Is there a way to add people to a conversation/pm thread? I see someone can remove themselves individually, but don’t see if adding is possible
If you started the convo, you should see Invite More under Conversation Participants. If you didn't start it, that option isn't there, so don't think you can add.
ETA: When you start a convo, you can check a box to allow other participants to invite others, but it's not the default setting.