When it's a party at someone's house, usually the host or hostess introduces people to each other. If that doesn't happen within a reasonable time, a guest may certainly walk up to another guest and say, "I'm John Jones! Joe Blow and I work together at Widgets." The other guest then replies, "Glad to meet you, John! I'm Tom Swift -- I know Joe from when we were both reporters for the newspaper."
The concept, as described in an etiquette book I once read, is "The roof is the introduction." In other words, you're both personal friends of the host or hostess, so presumably it's OK for you to get to know each other better.
However, if it's a big impersonal party, such as a charity event where people are clustered in groups at their own tables, or an alumni reunion where hundreds have showed up, it's OK to be more reserved. Sneak a peek at name tags, if any, and chat with the people you know. You can introduce them to other acquaintances of yours, if you like.