Both the guild I belong to currently and the one where I used to live, love to have new people drop in. My old guild is now charging $5 for guests, but they do provide refreshments at the meeting.
If you are uncomfortable just showing up, go to your LQS and ask them about the guild, usually someone who works there or someone in the store may be a member and can give you some insight. Also, see if the guild has a website that provides information about the guild.