So if i'm 15 and and i have a membership and i have a friend that's 13 but he doesn't have a membership can i bring him as a guest or does someone that's 18 have to be there?

You may want to call the gym and ask them-just to be sure you get the right information. This is something you might want to go to the source for to get the right info

The gym owner posted on another question that 15 is when you can be at the gym without an adult present.

I believe the minimum age to be on the main floor working out on the machines is 14.

Guest must be 18 years of age. Anyone under 15 must have their own parent there

Call the gym