An uninvited visitor; someone who turns up without being invited (noun). The verb form is '押しかける' (to show up uninvited).
週末に押しかけの客が来て、予定が台無しになった。
An uninvited guest came on the weekend and ruined our plans.
彼女は押しかけで訪ねてきて、帰る気配がない。
She turned up uninvited and shows no sign of leaving.
夜遅くに押しかけが来るのは迷惑なので、連絡してほしい。
Uninvited visitors coming late at night are a nuisance, so please contact me first.