How to merge/sum records by group using LINQ?

For example, how can I group the following records by GroupId using LINQ, and sum all other columns in each group? (thus merging all rows in each group into one)

var list = new List<Foo>()
{ 
    new Foo() { GroupId = 0, ValueA = 10, ValueB = 100 },
    new Foo() { GroupId = 1, ValueA = 30, ValueB = 700 },
    new Foo() { GroupId = 1, ValueA = 40, ValueB = 500 },
    new Foo() { GroupId = 2, ValueA = 80, ValueB = 300 },
    new Foo() { GroupId = 2, ValueA = 20, ValueB = 200 },
    new Foo() { GroupId = 2, ValueA = 20, ValueB = 200 }
};

Expected result is :

| GroupId | ValueA | ValueB |
|---------|--------|--------|
|    0    |   10   |   100  |
|    1    |   70   |   1200 |
|    2    |   120  |   700  |