Users can tap next to their avatar and name at the top of the navigation menu to access the account drop-down menu. To add a new account, users tap and they’ll be redirected to the familiar Salesforce1 login page to enter their user name and password credentials. Once their user name and password are authenticated, they can open the account drop-down menu to see and access their accounts. The items your users see in the navigation menu changes based on the selected account. To remove an account from the account drop-down menu, simply log out of that organization.
Users can view and switch between communities and their internal organization. The communities your users see and have access to changes based on the selected account. Members of your communities who have Customer Community, Partner Community, or other supported portal licenses can use the account drop-down menu to access and switch between their communities.
Communities in Salesforce1 work similarly to the full Salesforce site with some minor differences. See Communities in Salesforce1: Limits and Differences from the Full Salesforce Site in the Salesforce Help.