Does what you see when referring to the index include the witnesses or just the people getting married?
The GRO (General Register Office) Index of Marriages contains only the names of the bride and groom, not witnesses.
I expect this question arises from the fact that often more than 2 people have the same page number in the index.
For marriages from 1837 to 1852, you will find up to 8 names (4 marriages) per page of the register, and after 1852 only 4 names (2 marriages) per page. It is not usually possible to match up the bride and groom with information only from the index before 1911.
For Church of England marriages, people with the same page number had to have married in the same church.
As an example, in September quarter 1859 I find the following names:
Goodwin, Ellen, Southwell, vol 7b, p 409
Housley, Catharine, Southwell, vol 7b, p 409
Mason, William, Southwell, vol 7b, p 409
Smith, John, Southwell, vol 7b, p 409
Using parish registers I was able to confirm that William Mason and Catharine Housley were married at Wellow Parish Church on 9 Jul 1859, while John Smith and Ellen Goodwin were married there on 29 Aug 1859.