There are two ways to join a group when you have an invitation from a current member:
Using an Invite Link
When you receive the invite link, tap it
The app will open and guide you through the next steps
Using a Group Code
Open the app menu
Tap "Join"
Enter the group code or invite link when prompted
For Private Groups
If the group is private, you'll need to request membership after using either the invite link or code.
The group admin will review your request and approve or decline it. Once approved, you'll automatically become a member of the group.