When did people begin asking others to pray for them or join them in prayer? It probably began when Jesus said, "When two or more of you are gathered..."
Early Church Fathers (Eusebius being one) believed in the Assumption, as did many in the early Church. Some believe she died about 63 AD, and that all the Apostles (except James) were there. It appears the Assumption was talked about early on, so the argument over the Assumption can be whether it happened or whether it became an early legend. Church Fathers seemed to accept the Assumption, as while there was talk about it, there was no argument over it.