In order to get all of your colleagues (developers, product managers, legal, etc.) access to a Site, you need to invite them to join by their email address. Note:The same email address can work across many accounts and Sites! 

You (and your colleagues) can tell which Sites your email address can access by clicking the Change Site button (crossing arrows) near the bottom of the navigation toolbar on the left side of the screen.

Inviting a Member to your Site

To get a new member set up on your Site, go to your Site Settings and choose the Users option from the list on the left:

From here, you can invite users by clicking the circle with the plus sign in the lower right corner. The Invite User to Account window appears. 

Enter the email address of the Member you'd like to invite and choose their permission level. Select the each Site you want to invite them to and then click the Invite to # Site button. If this is the User's first Site, they'll log directly in to it, otherwise, they'll see the Site as an option when they click the Change Site button the next time they log in.