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Dim user As SPUser = SPContext.Current.Web.CurrentUser
Checking is current login is admin or not?
SPContext.Current.Web.CurrentUser.IsSiteAdmin
This will give, all group, which are present in current site
SPContext.Current.Web.Groups
This will give the group name of current user
SPContext.Current.Web.CurrentUser.Groups
Here i m getting all user and add in one DropDownList
Dim websp As SPWeb = SPContext.Current.Site.RootWeb
Dim users As SPUserCollection = websp.AllUsers
For Each i As SPUser In users
dropdownlist1.Items.Add(CStr(i.Name))