Anyone in a group can add their contacts as new members—you don't need to be the group admin. Before starting, make sure you have established contact on Session with the person you want to add (you've messaged them on Session, and they've accepted your message request). That way, you can be sure they'll appear in your contacts list.
iOS
1. Open the group
2. Tap the gear icon (⚙️) in the top-right corner
3. Select "Edit group"
4. Tap "Invite contacts"
5. Choose the contact(s) you want to add
6. Confirm your selection
Android
1. Open the group
2. Tap the three dots (⋮) in the top-right corner (Android)
3. Select "Edit group"
4. Tap "Add members"
5. Choose the contact(s) you want to add
6. Confirm your selection
Desktop
1. Open the group
2. Click the group icons in the top-right corner to open group settings
3. Look for the person icon with plus (+) in the top-right corner
4. Select the contact(s) you want to add
5. Click "OK" to confirm