You can only be a member of a single CrossFit gym in SugarWOD a time. If you belong to a few boxes, you can switch your gym each time you visit to log workouts and participate socially in each community.
You can find instructions on how to switch your gym in SugarWOD here.
Your workouts, PRs, etc come with you when you change gyms. You will not lose any data.
If you are a coach at an affiliate and you are needing to switch into another gym, your coaches verifications will be kicked off each time you switch. We recommend have an account for each location so you can keep your coach's permissions.