When you accept an email invite you are automatically given a username. Unfortunately it's not the best username for most people. No problem, it's quick and easy to change. Don't dally though as you have 3 days to change it yourself.
To change your username...
1) Click your avatar icon, top right, then your username or the gear icon.
2) Click preferences.
3) Change your username to something you like.
If you miss the 3 days grace period, no worries, send me a message and I'll do it for you.