Using your Google/Yahoo account to identify yourself will save you from having to remember your username and password for Task Messenger. Once your Task Messenger account is linked with your Google/Yahoo account, the next time you try to login to Task Messenger you can select the Log In With Google/Yahoo icon and we will send a request to Google/Yahoo asking who are you and then we will know which Task Messenger profile to load.
Yes, from the login page, select the appropriate "Login with .." button. Task Messenger will not find an account linked with your Google/Yahoo account, so you will have two option. 1) If you select the option on top of the page, you will be able to link your Task Messenger account to your Google/Yahoo account. This option will bring you to a new login screen. 2) You can create a new account using the register form that is on the loaded page.