There is no day of the business week (Mon-Fri) that when adding 5 days avoids hitting weekend days, so just add 7. This assumes that you are not allowed to begin on a weekend day (Sat-Sun), and that you don’t care about international dates (where they are a day ahead or behind).
Yes, adding 7 is a good solution. Another option may be to interrogate the day by Weekday function. Then add 1 or 2 days if it is on Sat or Sunday. This forces Sat or Sunday to next Monday. Hope it helps.
Example: Format cells B2, C2, and E2 as Date. B2 as 7/13/2008; C2 =B2+7;D2 =Weekday(C2); E2 as =IF(D2=1,C2 +1,IF(D2 =7, C2 +2,C2))