Here are Colombo places with communal/shared lounges or social common areas where it’s easy to meet other guests — a mix of social hostels (most reliably social) and a few hotels with public lounges/bars:

Hostels / social stays (best for meeting travellers)

• Seconds Colombo — boutique/eco hostel with multiple common areas, garden café, pool and a convivial lounge/restaurant that hosts events. (secondscolombo.com)
• Clock Inn Colombo (and Clock Inn Dehiwala) — budget hostel with a communal lounge/TV area, shared kitchen and regular traveler-friendly spaces. (clock-inn-colombo.hotelsincolombo.org)
• Ceylon StayZ — centrally located hostel with a shared lounge, communal kitchen and rooftop/communal spaces for meeting others. (ceylon-stayz.hotelsincolombo.net)
• Colombo Beach Hostel / Mount Lavinia (Colombo Mount Beach Hostel) — beachside hostel with rooftop/common lounge, shared kitchen and evening social vibe. (booking.com)
• Colombo City Hostel — rooftop chill/kitchen/lounge and regular social activities (good for solo travellers). (us.trip.com)
• Tuk Tuk Hostels — purpose-built social hostel (dorms + common areas, events like game/karaoke nights). (qantas.com)
• Lost & Found / other independent hostels — advertise cafés, courtyard common areas and in‑house hangout spaces. (lostandfoundsrilanka.com)

Hotels with public lounges / bars (more mixed social scene)

• Galle Face Hotel — historic beachfront hotel with multiple public dining/lounges and bars (Verandah, Travellers’ Bar) where guests and locals mingle. Good for afternoon tea / evening drinks. (gallefacehotel.com)
• Taj Samudra — large hotel with lobby bar/lounge, pool bar and public F&B venues where guests often socialize. (oyster.com)
• Hilton Colombo / Sheraton Colombo — big hotels with public bars, lobby areas and (for club rooms) executive lounges; public bars/lobbies are easiest places to meet other guests. (en.wikipedia.org)

Quick tips

• If your primary goal is to meet other travellers, choose hostels or boutique hostels (they run events, shared kitchens, rooftop bars, regular social programming). Hostels in Colombo are widely recommended for social travellers. (thebrokebackpacker.com)
• Big hotels do have public bars and lobby lounges (and sometimes evening events), but they’re usually less “meet‑everybody” than hostels. If you prefer a hotel, pick one with an active bar/rooftop or book into club/executive rooms that include shared lounge access. (en.wikipedia.org)
• Check current event listings / opening hours before you book (some lounges or group events change seasonally).

If you want, I can:

• Filter these by neighbourhood (Fort, Kollupitiya/Cinnamon Gardens, Mount Lavinia) or budget, or
• Pull current availability/prices for specific dates at any of the places above.