Exactly, except you forgot the last step which is:
Copy/Paste the contents of the new column directly into the input box in the Add Members page. And you must have an = sign before CONCATENATE. and missing comma before lncell.
(You do not need the spaces after the commas. You do need the space before the < symbol.)
If you're really new at Excel, here's some more detail. Assume that the first entry you have is in Row 2 with Firstname in A2, Lastname in B2, and email address in C2.
Then you would put =CONCATENATE(A2," ",B2, " <",C2,">") in D2.
Press Enter
Then select cell D2 and select Copy (or press Ctrl-C)
Now highlight from D3 downward to the last row of data.
Press Ctrl-V and the formula will be copied to all the Column D cells, and magically the references to A2, B2, and C2 will be updated to the appropriate rows.
Laurence Marks