You can't, it doesn't support substitutions. You can alter the language string within GJs language plugin, but you can't use [username] or anything of the sort. I suppose you could edit GJs template files at the below location and implement this though (sorry, can't help you with that).

components/com_comprofiler/plugin/user/plug_cbgroupjive/templates/

Is it possible to add info text in the pane (to show on the first line of the tab-areas) with the modules text area ?

I assume you're meaning the information pane to the left of a groups content. Yes and no; It can be done by editing the template or creating a new integration plugin. Aside from that no there is no easy 1 click addition or anything of the sort.