Check the settings of the group itself. By default the only person who can add users to a group is the person who created the group. I normally change the group settings so the group is owned by the Site collection root site owners group. That
way any of those users can modify the membership of the group.
Paul Stork SharePoint Server MVP
Principal Architect: Blue Chip Consulting Group
Blog: http://dontpapanic.com/blog
Twitter: Follow @pstork
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