Request to join
To join a specific team, select Teams > Join or create a team at the bottom of the teams list. Type a specific team name in the search box at the top right and press Enter. Find the team you're looking for and select Join team.
You can immediately join a public team, but will have to request to join a private team and wait for the owner's approval to access the team and its channels.
Note: If a team owner hasn't enabled discoverability of their private team, you won't be able to find it via search and will have to contact them directly to request to join.
Another option is to go to the command box at the top of Teams, then type /join to view a list of all your suggested teams. Select a team to request permission to join.
Request to add a member
To request someone be added to a team you're already a member of, go to the team in your team list then select More options > Add member. Type the name of one or more people then select Send request. Team owners will receive an alert that they have a pending request.
First, tap Teams at the bottom of the app, then Join or create a team in the top right corner. From there, you can create a new team, browse existing teams to join, or join a team with a code (if you have one).