Learn Hardcore Croatian: Ustani, molim te, jer žurimo. - Get up, please, because we are in a hurry.

Ustani is the 2nd person singular imperative of ustati (to get up/stand up). It addresses one person informally (a friend, child, someone you’re on a first-name basis with).

How do I say this to more than one person or politely to a stranger?

Use the plural/formal imperative and pronoun: Ustanite, molim vas, jer žurimo. This works both for addressing several people and for polite/formal singular.

What’s the difference between ustani and ustaj?

Both can translate as “get up,” but:

Ustani (from perfective ustati) is for a single, complete action: stand up now.
Ustaj (from imperfective ustajati) can sound like “(start) getting up” or a repeated/habitual command (e.g., every morning). For a one-off “Stand up,” Ustani is more common.

Can I use digni se/diži se instead of ustani?

Yes. Digni se (pf) and Diži se (impf) are common colloquial alternatives meaning “get up/stand up (from sitting/lying).” Ustani is slightly more neutral/standard; digni se can feel more informal.