Since it's a shotgun wedding, they're apparently going to have a low-key ceremony with only family and two college friends.
Literal
Shotgun-wedding [purpose-のため] family [and-と] university friends 2-people [only-しか] not-invite low-key-wedding [to-に] make [hearsay-そうです].
できちゃった婚 (shotgun wedding) is a colloquial term — できちゃった is the casual perfective of できる with the regretful ちゃった (contraction of てしまった), implying the pregnancy was unplanned. 地味婚 (low-key wedding) is an informal compound. しか~ない (only) emphasizes the small guest list. ~そうです conveys hearsay.