“Let’s Grab a Coffee” Means Getting One Casually

“Let’s grab a coffee” usually means “let’s go and have a coffee”. Here, grab does not describe clutching something with your hand. It means getting or having something in an easy, informal way, often without making elaborate plans. The expression sounds entirely natural in everyday Australian English, although it is not uniquely Australian.

After a meeting, for example, a colleague might ask, “Want to grab a coffee?” They may simply mean walking to a café, or they may be suggesting a brief chat over coffee. Grab makes the arrangement sound casual, but it does not promise that the meeting will be short. If you need to make a definite appointment, “Would you like to meet for coffee next Tuesday?” is clearer.

https://dictionary.cambridge.org/dictionary/english/grab
https://www.abc.net.au/news/2019-06-22/canberra-restaurant-kita-now-known-as-cop-cafe/11234036