You can join a token network by either:
Quick Connect lets you automatically open channels with 3 random online nodes.
If you click the "Transfer" button without having any open channels for the selected token you will get prompted to use Quick Connect.
When using Quick Connect:
Enter the amount of tokens you want to allocate and click "Connect".
The allocated tokens will be split with:
20% in each of the three channels
40% for whenever you are joining a channel that has been opened by another node.
The amount you choose when connecting is what will be available for making payments. You can always add more funds by depositing to a channel.
You are now ready to make the first payment!
If you want to join the network for a token and that token is not displayed in the list of tokens it might mean that it has not been registered.
To register a token yourself:
Click "Select network" in the tokens view.
Click "Add new network".
Enter the address of the token you'd like to register and click "Register".
You can follow the steps for Quick Connect to join the network of your newly registered token.