Which two expressions? There might be more than one way to accomplish what you want, depending on what's to be grouped.

The easiest way might be to create a Shape Expression which will contain more than one expression. The finished expression automatically groups them.

It took me a while to figure out how to add guitar string numbers (numbers inside circles). It might be helpful to add instructions for this to the index in the user manual.

Numbers in circles are in many fonts. Wingdings 1 and 2, Zapf Dingbats, and (I'm sure) many more. Just use them to create your fingering expressions.

Did you look in the manual for creating expressions?