Unable to add an user or a subgroup to a group on IE8

Steps to reproduce:

• Hover over "Wiki" and click on "Administer Wiki"
• Click on "Groups"
• Click on the small pencil icon to edit a group
• Write something in the "Users to add" and "Subgroups to add" fields

You'll notice a continous animation in the first field and that you are unable to write anything in it and also that the group suggest picker doesn't work for the second field. I've added screenshots.