How to select samples within a checkBoxGroupInput in R shiny

I am using a shiny application available at (https://github.com/jminnier/STARTapp). Using this application i am generating a heatmap from the Heatmaps page of the application (https://kcvi.shinyapps.io/START/) which is as shown below:

Here i would need to use only selected samples within a group in the heatmap. For example, from 'group1' i need to plot only 'group1_1' and 'group1_3' (in other words exclude 'group1_2') on the X-axis of the heatmap. Could someone help how to do this?

Also, there is a need to allow the user to see what are the samples within a group to select or exclude. i.e. the user should be able to know that 'group1_1', 'group1_2', 'group1_3'....'group1_100' samples exist in group1 that allows to select the desired samples.

Given my expertise, I realized that it is complex to create a minimal example of this alone and hence referring to the code in the github page. The corresponding code blocks exist in 'ui-tab-heatmap.R' and 'server-heatmap.R' at https://github.com/jminnier/STARTapp

Could someone hint a way to do this.