For our free version of Spark client, please search our support site to answer your questions and/or provide feedback via the feedback feature from within the client UI.
If you have paid version (aka using WebEx), you can call and open case with our Technical Support team.
For your issues with email in particular, ask user to check their "junk/spam" folder for email invitation or try to invite the same user from within the Spark client UI instead of WebEx UI.