Phone randomly added my and other contacts to group chat?

'Ive had a similar experience, my iPhone 4s is randomly adding and removing multiple random contacts in one of my group chat conversations. Ther is only one other person in the group chat (we are texting each other on multiple devices, iPhone 4s, iPhone 5c, iPhone 6, iPad 2 and iPad Air) but my iPhone is showing lots of adding and removing of multiple contacts from "my" contacts list, even at 4am when we're both asleep. The strange thing is that the adding and removing activity is only displayed on the iPhone 4s, I dont think that the other contacts are receiving the iMessages (haven't spoken to them in years and had no strange "why are you texting me that" messages) but it is strange behaviour.

Is there a question here?

If I had to, I'd hazard a guess that you're asking how/why your phone added you to a group chat? The simple answer is - it can't, and it didn't. If you're not part of a group chat then there's no way your phone even knows of the existence of the chat, and no way it can arbitrarily join.

On the other hand, if someone else in the conversation added you to the chat, well, that's a whole different question. I suggest you talk to the other people on the chat to find out who added you, and why.