Organize computers effectively. Line up the computers, so that there is enough space next to each computer for two or three students to sit together and work.
Divide students appropriately. Assign groups to parallel other classroom groups, such as their reading groups, or mix it up and give the students the opportunity to work with different classmates.
Change activities regularly. You might have computer games that your school board recommends, or you can use sites like Starfall.com or Abcya.com to search for age-appropriate and unit-appropriate activities for your students.
Assign centers, so that students know where they are supposed to go. If your classroom has several groups, give a color or animal name to each group. On a chart or dry erase board, you can indicate where each group should go.