You can add any user on a Site to any Group on the same Site. You cannot add users from one Site to another Site.
If a user does not have individual permissions for a folder and is a member of more than one Group, the Server gives the user the least-restrictive access for the folder. You can individually modify user permissions and those modified permissions will outweigh all Group permissions. For example, if a user is a member of three Groups that all have upload permissions to a particular folder, but you have denied that specific user permission to upload to the folder, then the user cannot upload to the folder.
To move users into or out of a group
In the Administrator, connect to the server, then click the Server tab.
In the left pane, click the Group you want to configure. The Group Membership tab appears.
In the right pane, double-click the user or use the arrows to move the user into or out of the Group. (You can multi-select using SHIFT and CTRL.)