Can I see people that are immediately available?

We don't have a filter for that at this stage. For the most part, the devs on OfferZen are currently employed and have to work off their notice period.
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If a dev is available immediately we normally say so in their short summary you see on their "card" on the list. So a "hack" could be to search in your browser (cntrl/cmd f) for the word immediately.