XFGetCell for multiple members

Is there a way, or a best practice recommended, to point an XFGetCell formula to one or multiple members of the same dimension without adding parent level members in the dimension hierarchy?

As an example, if we have entities 1 through 5, I'd like to be able to change an input cell to any combination of those and have an XFGetCell function to pull in the sum of all the data points referenced. Currently I'm thinking this would require nested If functions, but that seems like it would quickly get to an unmanageable length as the number of members grows.