To join a guild, you need to tap the Join button in the Guild window.

How to open the Guild window:

Option 1
1. Tap the name of a guild in the Join Guild tab if you are not already a member of any guild

Option 2
1. In the Join Guild tab, tap the Search button
2. In the window that opens, enter the name or ID of the guild
3. Tap the Search button
4. Tap any guild name in the search results

Option 3
1. In the Friends tab, tap a friend's name
2. Tap the name of the guild they are a member of.

Important!
Please keep in mind that you won't be able to join a guild if it's full or if your level doesn't meet the guild's requirements. Also, to join a closed guild, your request must be approved.