Should be pretty straight forward, just joining the new combatant to the player group and battle group (happens automatically if there's a free slot in the battle group, otherwise via a Group menu screen) is enough - e.g. as in the game tutorial 50.
I suspect your combatants are set to be AI controlled - check their Battle Settings > AI Settings, AI Controlled should be disabled if you want the player to select actions for them.
Keep in mind that all non-player combatants are AI controlled anyway, so the AI control settings are purely for player group members.