The 30,000 or so invited guests to the opening of the Clinton Presidential Library are arriving at the grounds. The library and museum east of downtown Little Rock is the center of the universe for the Democratic Party today, with numerous former Clinton administration officials and other power brokers on the guest list. President Bush is to speak at the ceremonies that begin at 10 a.m. Security is tight and guests are finding plenty of items on the prohibited list. Among the things that ticket holders can`t bring are umbrellas. They`ll have to make due with ponchos and rain coats in the wet weather.