Translation Memory sharing is only available on the Advanced plan and up.
Each project in Transifex has its own Translation Memory. However, if you have projects which have similar or related content, e.g. a project for your iOS app, and one for your Android app, you can share TMs across those projects by creating a TM group.
To share TM:
Head to your Organization Settings from the main navigation.
On the left menu, click on Translation Memory.
Click on Add Translation Memory Group.
In the popup, give your TM group a name.
Select which projects to add to the TM group by clicking on + icon next to the project name. Projects in the same TM group must share the same source language.
Click Apply to finish creating the TM group.
Things to note about Translation Memory groups
A project can only belong to one TM group.
A new TM is not formed when you create a TM group; TMs are just shared amongst projects in the group.