If you don't have a pair of split ring openers, use two pairs of chain-nose pliers instead. Position the pliers on each bar, then gently pull the bars in opposite directions until there is enough of an opening to pass the beads around the rings. When the beads are all in place, compress the ring with your chain-nose pliers back to its original shape. This will only work with smaller beads, like seed beads, as larger beads will be difficult to get past the curves of the safety pin ring.
Another option is to use the Pincharming® safety pins. These safety pins have a removable head that unscrews from the pin bar. Unscrew the head and remove it from the safety pin, place your beads onto the bar and then screw the head back on.