-s1- I have been joining in at the gym (wrong)
-s2- I have been working out at the gym (right)
-i1- I am not sure why s1 is wrong. I think may be because of the “at” particle (In this case “I have been joining in the gym” would be correct). Please could you explain or confirm?
-i2- join in means to participate. Is this right?
-i3- work out means to do exercise, right?
Thanks in advance !!