I often ask DNA cousins on 23andme to upload their raw data and their AF (Ancestry Finder) to Gedmatch.
Some of the benefits of doing this are:
-Ability to compare different people to each other. This isnt possible on 23andme. You can compare yourself to Cousin A or Cousin B, but not Cousin A to Cousin B.
-Ability to triangulate matches. When people upload their AF you can triangulate matches (see who you both match in AF)
-Ability to compare with people from FTDNA and 23andme.
-Ability to compare with matches for segments in common smaller than 5 cM. It is debatable if this is much use genealogically, but sometimes you want to see if a segment was just under the 5cM threshold.
How to upload your 23andme data to GEDMATCH:
First download your raw data. To do this you go to "Account" in the top right of any 23andme page. Then select "Browse Raw Data". Then in the top right will be an option to "download raw data".
You will have to enter your password and your secret question answer. Then you will have a file, named something like "genome_your_name_full_date"
Then on the homepage of Gedmatch, scroll down to "UPLOAD YOUR DATA FILES", and under that to "upload your 23andme raw data file", click the link and follow the instructions. Record the # given to you, usually an M and some numbers. Consider putting this # on your 23andme profile.
Next, go back to 23andme and get your AF file.
First go to "Ancestry File" then to "Ancestry Finder". I assume here that you have filled in Ancestry Finder. If not, then do this now. Even if you have to put "unknown" for every location of parents and grandparents birth. Near the bottom of the page is "Download Your Name's Ancestry Finder Matches". Then you will get another file named something like "ancestry_finder_your_name_full_date"
Then back to Gedmatch, and this time under the "UPLOAD YOUR DATA FILES" go to "Upload your 23andme Ancestry Finder File" and follow the instructions there.