Google Sheets Function: RANK
The RANK function returns the rank of a value based on a set of values (for example, to define a ranking).
Usage:
=RANK(value, data, is_ascending)
Usage Example
The RANK function here should return the ranking of each user based on their score:

Enter into the RANK function:
- value: the value for which to determine the rank
- data: the range of cells containing all the values
- is_ascending: 0 for a descending ranking or 1 for an ascending ranking
The formula is therefore (do not forget the $ to copy the formula down):
=RANK(B2,$B$2:$B$11,0)

Users 4 and 10 normally occupy the 6th and 7th places but since they obtained the same score, they both occupy the sixth place (thus rank 7 was not assigned).
If needed, you can copy the Google Sheets document (or view the document) with this example.