I know everyone loves Rick, but he has consistently made some stupid decisions as leader.
After Father Gabriel is kidnapped by the trash people, Rick and the others meet them for the first time. These scavengers look and talk (who, apparently, can't even speak in complete sentences! -- after only what two or three years at this point?) like rejects from Mad Max central casting. Rick makes a deal to bring them a lot of guns.
After finding guns (what, maybe two weeks later?), Rick brings them to the trash people's leader, and she asks: "Are they operational?" rather than "Guns work-work?" Yet somehow this sudden Hooked-on-Phonics-like improvement in their apparent literacy doesn't raise any red-flags for Rick?
And then he invites them, a bunch of now heavily-armed strangers, inside Alexandria's walls?