Running missions with an NPC corp raises your standings with them. If no one else in your corp has even run missions for that NPC corp, the standings of your corp with that NPC corp are essentially your personal standings (minus social skills bonus).
In my case, this caused corp standings to rise over 8.0. So now other corp members can get jump clones with that npc corp. However, L4 mission agents for that npc corp still won't talk to my corp members. What am I missing here? How do I get the corp standings to allow my corpmates to get missions fom the agents without grinding for personal standings?