Updated:

1. Make the teams (once)

/team add seekers "Seekers"
/team modify seekers color red
/team add hiders "Hiders"
/team modify hiders nametagVisibility never
/scoreboard objectives add timer dummy
/bossbar add minecraft:hideseek "Time left"

Player name tags normally show through walls. nametagVisibility never hides the hiders' names. More in the team guide.

2. Put players on teams

/team join seekers Steve
/team join hiders @a[team=]

The first line picks the seeker (replace Steve). The second puts everyone without a team on the hiders. For a random seeker, run /team join seekers @r first.

3. Start

/gamemode adventure @a
/effect give @a[team=seekers] minecraft:blindness 30 0 true
/effect give @a[team=seekers] minecraft:slowness 30 6 true
/scoreboard players set #game timer 6000
/bossbar set minecraft:hideseek max 6000
/bossbar set minecraft:hideseek players @a

The seeker is blinded and slowed for 30 seconds while everyone hides. Adventure mode stops anyone digging through walls. 6000 is 5 minutes (20 per second).

4. Count down and show the timer

Put these in a repeating command block (Always Active) followed by chain command blocks:

/execute if score #game timer matches 1.. run scoreboard players remove #game timer 1
/execute store result bossbar minecraft:hideseek value run scoreboard players get #game timer
/execute if score #game timer matches 1200 run effect give @a[team=hiders] minecraft:glowing 60 0 true
/execute if score #game timer matches 1 run title @a title {"text":"Time's up!","color":"gold"}

The timer drops by 1 each tick and the boss bar follows it. With one minute left (1200), hiders start glowing through walls. See the bossbar guide and glowing guide.

5. Found players

/gamemode spectator Alex
/team join seekers Alex

Switch found players to spectator, or move them to the seekers so they help search.

6. Clean up

/bossbar set minecraft:hideseek players
/effect clear @a
/team leave @a
/gamemode survival @a

This hides the boss bar and resets effects and teams. To play again, start from step 2.

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