Once you have logged into the Forefront Identity Manager online interface, you must choose the group you would like to manage.
To see all groups you manage, click on Manage my SGs
You will then be presented with a list of groups for which you are listed as a manager, as well as several options along the top of the screen.
To add a member to a group or groups you manage, click the check box next to the name of the group(s) you would like to modify, and then click on the Add Member icon above. 
A new window will pop up, with an empty box to add members above, and below a semi-colon separated list of groups the people will be added to. Enter the names or netids of the people you would like to add to the group, separated by a semicolon.