formula field issues

Is there a way to make a formula field calculate properly if one of more of the items it's supposed to sum are blank? For example, say I have three number fields: Group A invitations, Group B invitations, and Group C invitations. If the formula field expression is (Group_A_invitations + Group_B_invitations + Group_C_invitations). but there's no value for Group C because no one from that group was invited, then the formula won't work. I still want the sum of groups A and B though. How can I make this work?