How to add commas to output?

Question

bash

Good day!
Please tell me how to correctly add a command in bash in order to get a list of groups, of which $USER is a member, separated by commas without spaces.
Now I receive

groups $USER
test everyone netac com.app

Need something like this

#команда
root,admin,master,nours,boca

Tried cat grep and the -m switch, didn't work.
Thank you for attention!

Answer 1

groups $USER | tr ' ' ','

Answer 2

groups $USER | tr ' ' ',' | cut -d, -f3-