I inadvertently "left the conversation " of a text. How do I get back on? Iphone 15+

I had to call Verizon support. The tech got onto my phone and saw what was there. Believe something was tweaked on their end to make the issue vanish. Miracles do happen.

also, my sister said she added me and got a reply from me, but my phone does not show that that happened

I started new conversation with this group, than turned my phone off thinking that might set thingds right. instead those conversations merged and i cannot text on thread