I have added the plink tag to your question. Also, to help, please edit your question to show the commands that you have used and the error messages received.

Finally, I presume that you have already read this: https://www.cog-genomics.org/plink/1.9/data#update_indiv

Here is some information about what I have done so far:

I convert our 23andme files to .bed format with the following command:

plink --23file 1.txt Person1 AGENT M --out 1

plink --23file 2.txt Person2 AGENT M --out 2

plink --23file 3.txt Person3 AGENT M --out 3

so on...

This seems to work fine. Then I merge with other files similarly created using the following command:

plink --bfile 1--merge-list list.txt --make-bed --out CombinedPerson

The file list.txt is formatted as follows for simplification:

2.bed 2.bim 2.fam

3.bed 3.bim 3.fam

4.bed 4.bim 4.fam

so on....

This also seems to work, but the software I put the CombinedPerson file through leads me to believe a small mess was created.

Warning: Multiple positions seen for variant 'rs4477212'.

Warning: Multiple positions seen for variant 'rs3094315'.

Warning: Multiple positions seen for variant 'rs3131972'.

1920500 more multiple-position warnings: see log file.