Every Space you're a member of is listed under Spaces in the sidebar, in alphabetical order. Public Spaces you haven't joined are in Search.
Open one of your Spaces
In the sidebar, find the Spaces heading. If the sidebar shows only icons, expand it with the panel button at the top.
Select a Space's name. It opens on its first tab, usually Feed.
What's on the list
Spaces you joined or were added to, in alphabetical order rather than by recent activity.
Private Spaces have a padlock next to their name.
The list updates straight away when you join or leave a Space. If you aren't in any, it shows No spaces yet.
Find a Space you haven't joined
Public Spaces you haven't joined aren't in the sidebar. From any page other than Home, select Search in the sidebar, type the Space's name and look under Spaces in the results.
Private Spaces only appear once an owner or moderator adds you. You'll get a notification when they do.
Good to know
The + next to the Spaces heading creates a new Space. It isn't shown to people with Read Only access.
A Space you expected isn't listed? You may have left it, been removed, or it may be private.