Energy work can be very draining, and it's important to have plenty of substantial food available. If you are drunk, stoned, or on strong prescription drugs, you do not belong in circle. These things can disrupt your energy pattern and interfere with circle work. The exception to this is if you need to take prescription medicine to regulate your mood or personality. If you want to see how a particular group works, contact them to find out the procedure for visiting one of their events.
Some groups require sponsorship for guests.
Try to contribute to the needs of the group by bringing a potluck dish, materials like candles or whatever, etc. Some groups expect you to do so, some do not. Check with your group to learn proper protocol for identifying members, discussing group business with non-members, etc. Some are very secretive about membership, locations of rituals, etc. Never, ever give away names or contact phone numbers or addresses of members in the group without their express permission on a case by case basis.