You can mention another member in a Community post or Comment by typing the @ button and then typing and selecting the name of the member you wish to mention. This sends them a notification and a link to the post where they can join the conversation. To mention someone, they must already be a member of the Community. Otherwise you may wish to invite them first using the "Invite Colleagues" button.
We do not currently have the ability to DM or follow another user on the website, but that will be coming soon.