=VLOOKUP("*"&G3&"*",$B$3:$C$193,2,FALSE)
Translation:
look for a match on the contents of G3,
across the range of cells from B3 to C193 (B has occurrences of what we want, C has the value we want to print in THIS cell)
and if you find a match, print the 2nd column's value,
and don't force an exact match because these are text strings instead of numbers.
This assumes first names in A2 and last names in B2, and generates first initial + lastname usernames.
=LOWER(CONCATENATE(LEFT(A2,1),(SUBSTITUTE(SUBSTITUTE(SUBSTITUTE(SUBSTITUTE(B2," ",""),"-",""),"'",""),".",""))))