In order to invite a user to a project and provide view-only permissions, you can follow the steps below:
Let's take, for example, a project with source language English (Canada) and target language French.
Visit the team's page and click on the "Invite Collaborators" button at the upper right corner of the page.
Provide the tx username of the user you want to invite to the team (the email address can also be used).
Select the role of the user in the team (you need to select Language coordinator, Reviewer or Translator - the role of the user in a source language group does not really matter so you can select any of the above options).
Then find the translation team that has been assigned to the specific project in Transifex.
Finally, select the source language (en_CA) of the project from the drop down menu.
Once the user is invited, he will be able to only view strings online. In order to do that, he needs to click on the source language of the project and then "View strings online" as follows:
The list of the strings will be accessible to him as well as the option to see translations in other languages without being able to modify any translations there though:
If the user tries to click on other languages of the project, the following message will appear preventing him from entering the editor mode: