It’s not possible to join a team trial account (accept the e-mail invitation), if you currently have a Premium subscription.
However, you can create a new account and join the team using the new account instead of your current one. In this case, you’ll need to ask the team administrator to send you a new invitation.
Another option is to change your username (for the current account). After you do that, you will be able to accept the team invitation with the original email as a new account.
Log in and click Change Email in the right-hand panel.
From the top left menu choose Account.
Click Change email.
Navigate to Settings.
Tap the name of your current plan at the top (iOS) or your email (Android).
Choose Change email.