Know Your Limits
When dealing with a group activity, it's important to know what you can or cannot do. If three of your friends want to go rock climbing, but you have never tried it, be honest. You may hold them back and make it a poor experience for everyone.
Privately, it's important to know your limits so you can push them. Maybe you have never ran a marathon before, but if you know you struggle running once around the neighborhood, push the envelope. Try and run one lap plus some more to improve yourself.