If someone casts a creature in their second main phase, after it resolves, is there time for another player to cast something before the current players end step? In other words, does the current player have to pass priority before moving to a new phase?
Yes, there is still time. After a spell resolves during the main phase, that doesn't end that main phase; rather, the active player gets priority (C.R. 117.3b). That player (and every other player) will still have priority to cast instant spells and activate abilities before that phase ends (C.R. 500.2, 117.1a-b, 117.3d). And a player can't end the main phase unilaterally, but rather, in general, the main phase ends only when all players agree (C.R. 500.2, 505.5).
The foregoing applies not only to a player's second main phase, but to any other main phase and any other step or phase where players get priority (C.R. 500.2, 505.5). See also this thread (declare attackers step) and this thread (beginning of combat step).
The foregoing applies not only to a player's second main phase, but to any other main phase and any other step or phase where players get priority (C.R. 500.2, 505.5). See also this thread (declare attackers step) and this thread (beginning of combat step).
EDIT (Jun. 14): Clarification.