I have email working from the ELK/XEP but was wondering if there was a way to send the user name that armed/disarmed the system...
In short, I don't think so.
Well, you might
be able to setup a separate "email" for each person and then use rules, like
Whenever Area 1 Arm state becomes armed
and last user was user 1
then send email1
and do a rule per person. But, that is, presuming the last user bit even works, a kludge.
There's no variable substitution available on the email messages, thought that's a feature that's been suggested almost since the XEP came out. Unless Elk has some great new things awaiting us in the next release of the XEP firmware, this type of thing is currently best done via an attached PC running more powerful software.