To be able to complete these Dispatch Missions effectively you will have to recruit additional agents either by creating them in the Agent Roster Menu or later on by getting them as mission rewards.
The different Dispatch Missions have requirement levels which you will have to match with your agents' character levels.
Example:
Mission level 7 --> One Lvl. 3 agent + two Lvl. 2 agents
Your agents will get some sweet XP during these missions so you can pull them up even when you don't take them with you in the field.