gate crasher

I came with a friend who knew

one of the gate crashers.

Gate crashing, gatecrashing or party crashing is the act of attending an invite-only event when not invited.

The person doing the gate crashing is known as a gate crasher.

Oh, boy, you really loused up that toast.
louse up [louse sth⇔up] phr v
to make something worse rather than better, or to spoil something