I would like to calculate the mobility of a water and ion solution. One method I would like to do this is to implement an electric field in the system and calculate the drift velocity (velocity not including the thermal velocity) of the ions. What is a good method of calculating the drift velocity? I couldn’t find any existing functions in the LAMMPS documentation to do this.

Additionally, how does the boundary conditions work when an electric field is implemented?

I would like to calculate the mobility of a water and ion solution. One
method I would like to do this is to implement an electric field in the
system and calculate the drift velocity (velocity not including the thermal
velocity) of the ions. What is a good method of calculating the drift
velocity? I couldn't find any existing functions in the LAMMPS
documentation to do this.

have a more careful look at the documentation for the variable command.
check out "vcm()"

Additionally, how does the boundary conditions work when an electric field
is implemented?

boundaries don't matter for the model implemented in fix efield.