UPDATE: There are comments pointing out that this achievement may be bugged. An update may come soon, fixing it.
In order to unlock this, you have to join the game of someone who is in your party. Of course, he must be playing ROBLOX.
So, look for someone in your friends' list that is playing ROBLOX and send him an invitation to your party. Once he accepts the invite, click on his profile and choose join game (it is better to have the game quit before doing this).
Once the game is loaded, the achievement should pop.
As stated by
BADW0LF 01, if you don't have someone in your friend list playing ROBLOX, you can still achieve this with a local dummy account. Just sign in the dummy account, start ROBLOX, sign in with the main account and then invite the dummy account to a party. After the party invite is accepted, start a game on ROBLOX with the main account for the achievement to pop.
Skelzorge was kind enough to make a step-by-step guide for the dummy account guide. Here it is:
1. Make sure your main and alt account are on each other's friends list
2. Sign out of your main account
3. Sign in your alt account
4. Launch ROBLOX
5. Create a ROBLOX account on your alt account (if you haven't done so already)
6, Load up a game in ROBLOX and wait until you spawn
7. Sign in your main account
8. Invite your alt account to your party (your alt account will automatically join the party)
9. Launch ROBLOX on your main account
10. Load up a game in ROBLOX with your main account and wait until you spawn