Membergroups - How do I change a member's membergroup From Online Manual
There are two ways to change a members membergroup: One way is through their profile, the other way is through the edit membergroups page.
To edit the membergroup through the members profile, go to members profile > Modify Profile > Account Settings. If you do not see this make sure you are logged in as an administrator. Near the middle of the page there should be a drop down list with the members current membergroup selected. Change the box to display the desired membergroup and select "change profile" to save. You can also assign additional membergroups by selecting the "show additional groups" link below the selection box.
To edit the membergroup through the Administration center, go to the membergroup section (Administration Center > Members > Membergroups > Edit Membergroups) and select the name of the membergroup you want to place the member in. Here you can add members you want in the membergroup by typing their username in the "List of Members to Add" input and then selecting the "Add Members" button.
Note: You cannot assign a person as a local moderator (moderator of a specific board) through either of these means. See the FAQ How do I assign members as local moderators for more information about moderators. This only works for membergroups, you cannot assign members to post count based groups.