There are two ways to add friends. You can by clicking on your friends list in the lower left hand corner of the screen and enter their Glyph Tag.
Alternatively you can use the chat and enter the command /friend add Glyph#Tag. This can be done in the lobby or during a match.
Where can I see my Glyph Tag?
Your Glyph Tag will show up on the Glyph launcher in the top right hand corner and is also visible when you open up your Atlas Reactor friends list. Glyph Tags do not need to be unique. Each Glyph Tag is assigned a numeric ID to create a unique identifier (for example, Trion#0311).