Short answer: Use the menu in chat or on the profile and pick Add Friend.
From Messages
- 💬 Open the conversation.
- ⋮ Tap the three dots.
- ➕ Choose Save as Friend/Favorite → Add Friend (request is sent).
From Profile
- 👤 Open the member’s profile.
- 💾 Tap Save → Add Friend.
- ✅ Optional: toggle Secret gallery, Know personally, Show nickname.
💡 Tip: If they’ve never messaged you, you may need a reply before you can add them.