Comparing columns in two excel files

Hello,

I have read through many of the solutions here, but cannot find one that works for me.

I have two excel files containing different data with different number of columns from two different systems. Each file also has a different number of records (rows)
The only thing in common in both files is a username.
I wish to compare the two files to identify which username appears on both systems. I would like to create a formula that identifies the members of both groups.
I have tried dget and vlookup but not finding any matches, yet I can see that there are matches there.
Is there a way to do this please?