there isnt really a solution for this problem right now if you want to contact this person in the Steemitspace.
I predict "private" messaging between steemit users will be an available tool after Beta state.
But of course there are external tools you could use.

for example
https://steemit.chat/home
https://steem.slack.com/

or you could ask for the emailadress.

for now you unfortunately have to manage it with this.