 ''This is a mistake - a serious, serious mistake,'' said Sharon Polster, a resident of St. Charles, who led a meeting of the newly created Coalition Against Concealed Guns. The meeting was held at the South City YMCA.  ''I'm mad; I'm damned mad,'' said Gil Spieldoch of Creve Coeur, vice president of Missouri Council for the Control of Handguns and one of the organizers of the new group.  ''We have got to mobilize that anger,'' said Spieldoch. ''We've got two things going for us. We've got the numbers, and we're right.'' The new group, which includes several victims of gun violence, anti-handgun activists and representatives from social service agencies and the League of Women Voters, said Sunday they plan a bus trip to Jefferson City in the next two weeks to lobby against the concealed-weapons proposal.  The group also plans a telephone appeal to legislators to try to get them to vote against the proposal. 