I have the overseers office fully upgraded but I'm not sure how to send them out on more than one quest at a time. Thanks in advance!
It depends on what level your Overseer's Office is. At level 1 you can only send out one group at a time. At level 2, you can send 2 groups. At level 3, you can send 3 groups.
Once you have levelled up your Overseer office, you can then select a mission, send out the group, then repeat the process for the next group.
 : Source