Systems
Next to the Join Group sign there's a chest with a screen above it.
The chest's screen lists the group benefits:
The claim prompt says "Claim x2 Gains for 60 minutes?" for a player with all three extras (15 + 15 + 15 + 15 minutes).
Sources
Parts of this wiki use information from the OP Ninja Simulator Roblox Wiki on Fandom, by its contributors, licensed under CC BY-SA. Text adapted from it is shared under the same license; each page lists its sources. A big thank-you to Deadshot123def456 and 1mposs1bl6 for keeping the Fandom wiki up to date.