There is no “right” answer to this one. You know when you walk into a group of people sometimes if this will be a good fit for you and if you will eventually join. Some groups that have been around for years have very strict and written-out guidelines which are adhered to…period. Others are much more laid back and informal. Sometimes it depends on the type of business you have; sometimes it depends upon the type of person you are, but rest assured, there is a group out there for everyone. You will learn quickly that even “after hours” events are different and appeal to a variety of tastes.
For example, I have attended after-hours events where there is barely a sober person left in the joint, and I have attended black tie after hours events with upwards of 900 people where the food was served on trays by perfectly attired wait staff. Make sure you research where you are going and always have a plan, even for these events. You may have a chance to look, via the internet, at the attendee list prior to showing up. That is a bonus, to be sure. Use that information to your extreme advantage and if there is a VIP who will be attending, do a little info check on that person so you can be quite the conversationalist if you get to meet him or her in person.
It is the funniest thing to me that someone who relies mostly on networking to market themselves and/or their business that they do not carry business cards with them at all times! You simply never know when you will meet someone, strike up that conversation and both of you realize that they need your product or service and you do not have a business card with you. What is wrong with that picture?
Whether your group is strict or not, or whether you are attending a meeting during business hours or after business hours, be sure you are prepared to talk business if the need presents itself. Too much to eat or drink is never a great idea, and when you are out, be sure to have a way for people to contact you. Once everyone gets home or back to the office, without that contact information, it will be difficult to follow up.