There is no limit for the amount of users that can use one Muse headband. Each user would just have to set up individual Muse accounts on their mobile devices or tablets.
If you are using one mobile device or tablet, you would just need to each sign into/out of your Muse account before using Muse. To sign out of the app click on the 'settings' tab and scroll all the way to the bottom of the screen then click on 'sign out'.
The reasoning for setting up multiple accounts, is that one singular account used by multiple people, would cause the brain model data to be off and skew each persons results.