Being in multiple Slack teams simultaneously
While each team account is separate, you can use the same email address to sign up for multiple teams.
You can be signed in to multiple Slack teams at once, and when you sign out of one team, it doesn't affect your accounts on other teams.
--- Mac desktop app ---
The Mac desktop app (version 1.0+) allows you to seamlessly navigate between your teams.
Locate the Team Menu — click the down arrow next to your current team's name. Select Sign in to another team. Enter an existing team's domain and click Continue, or Create a new team. Enter your email address and password to sign in. Once you're signed in to more than one Slack team, the Mac app will display your team icons on the left-hand side so you can switch between them easily. (Owners and admins can customize this icon for their team: Uploading a team icon.)
--- Web ---
If you're using Slack on the web, you can open each team in a separate browser tab — just enter teamname.slack.com in your browser's address bar to sign in. You'll be able to receive desktop notifications for all teams simultaneously.
If you prefer to use a single browser tab, visit the Team Menu and select Sign in to another team. You'll then be able to switch between teams from this menu.
--- iOS & Android apps ---
Sign in to an additional team at any time via the Switch Team or Settings screens — use the menu button in the top right corner or swipe left. On iOS, you can also triple-swipe to switch quickly!
https://slack.zendesk.com/hc/en-us/articles/201405046-Joining-multiple-Slack-teams
You don't need to open more than one instance of the app, FYI. While it's running, you can go to Window -> Sign in to another team... And sign in to another team simultaneously. I presently have 4 teams open in one slack window. Its really nice because it adds a new ~50px sidebar on the left displaying your teams in any order you choose (you can drag them around).
You can run open -n /Applications/Slack.app to open another instance.
There is a Beta to run in a single version of the native app which is available here, but I take no responsibility for its security :)