First, get access to Spotify for Artists. Then use Add team to join other artist or label teams:

Web browser

1. Log in to artists.spotify.com.
2. Click the 3 dots in the top-left.
3. Select Your teams.
4. Click ADD TEAM.

App

1. Log in to Spotify for Artists.
2. Tap your artist image at the top.
3. Tap Request access to a team.