OK, for anyone who is having the same challenge I will leave the example up (see above) and include the solution.
The way to overcome not being able to reference the group within a repeating group directly.
Set a custom state on the repeating group within the main group, so every time you click on a link or button it sets the custom Yes/No state to “Yes”. It’s probably not necessary but on page load I also set the custom state to “No” - when you click the toggle button as part of the Workflow it first “Toggles” the view to visible and it then sets the state to “Yes” and then Bubble loads the Mentions plugin when the multi line text field is visible based on the value of the custom state. The work-flow then waits 500 ms before resetting the state to “No” ready for the next repeating group.
Most obvious option below:
In most Instances when you simply want to hide a field and then enable mentions you can just put it in the work flow after the Toggle option in the text field. In most cases this will work! Use the above if you need to associate it with something else.
Also don’t forget to add in the required coloured options in the plugin settings as @gaurav mentions above.