Count how many students you have in your class. Cut two squares of paper for every person and number them; use each number twice. For example, number two squares of paper with the number "1" for your first student, two more squares with the number "2" for the next student, and so forth. Hand one square of paper with a number to each student and put the copies in a bag. Tell each student that the number they have is their permanent number. Record these numbers so that you will have a record.
Mix up the bag of numbers and pull out four or five at a time. Mixing up the numbers and pulling them at random helps create diversity among groups. Diversity is particularly important when your class features students that speak the same native language such as Spanish or French. Often, while learning English, students have a desire to revert to speaking their native language with other native speakers. However, this is harder to do when placed in groups that feature students from different language backgrounds.
Repeat the process of mixing up the numbers in the bag and drawing them out each day you want students in reading groups. Placing students in new groups each time ensures that students grow comfortable speaking and reading in front of different people. Randomly picking groups also helps eliminate dominant roles within a permanent group. When dominant roles occur in permanent groups, they take away from the learning process of other less outspoken members of the group, which takes away from the learning experience. Keeping groups random also helps reduce stress among groups that feature students that do not necessarily get along.