If you're currently on a free Business Trial of MeisterTask, please note the following limitation:

• During the 14-day trial period, project sharing is limited to users within your own team.
• This means you can only invite users who are part of your Business Trial team. Sharing with individuals outside your team—such as someone on a separate Pro or Basic plan—is not supported during the trial phase.

How to check who's in your team:

• Go to Account Menu > My Team > Team Members in MeisterTask.

Only the people listed here can be invited to projects during the trial.

Workaround:

To collaborate with someone outside your trial team, you can:

• Ask them to join your trial team (this may require admin approval).

Or wait until you're on a paid Business plan, which enables external sharing and guest roles.