Can I give advice to other players in the group, or ask their caddies what club they hit?

No. Only the player you're caddying for may get advice from you, and your player may ask for advice only from you. During the round your player must not give advice to anyone else in the competition who is playing on the course, ask anyone other than you for advice, or touch another player's equipment (such as their bag) to find out something that would be advice. Because your actions count as your player's, the same limits apply to you.

Penalty: two strokes for your player in stroke play (loss of hole in match play).

Allowed: sharing public information, such as where the hole, a penalty area or another ball is, distances, wind direction and the Rules. In partner formats (four-ball, foursomes), you may also advise your player's partner.

Source: Rules of Golf (2023), Rules 10.2a, 10.3c and Definition of Advice.