Will a guest be counted twice if they are added as a new member?

The best practice is to either count someone as a Guest or Add them as a Member of the group, but do not do both.

If a leader adds a new member to a particular meeting as well as counting them in the guest count box in the meeting report, the default will be to count them as a member for that meeting, and the guest count is added in.  So, if they count them as a guest as well, they will be counted twice.  

Typically, if people are entered as a number in the guest box, then their names are just included in the meeting notes.  Once they are added to the group as members they are no longer considered guests to that particular group.