The platoon organization for Mechanized Infantry units equipped with the M2 Bradley IFV hasn't changed since the Force XXI restructure ("new" F-series TOEs). This organization was also included in the Limited Conversion Divisions. The original Bradley TOEs (J-series and the first Army of Excellence L-series TOE) were different.
In answer to the original question: they don't. The organization for mounted operations is generally different from that of dismounted operations. To emphasize this point, in the original J-series TOE, the weapons element was never intended to operate with the rest of the platoon during mounted operations (the weapons element was subsequently deleted entirely from the L-series TOE onward, making the mechanized infantry platoon the only one in the Army to not have a weapons squad).
In the field, attrition makes it easier as well (which is probably why the Vehicle Section is authorized one spare gunner too). The troop capacity of the M2A2 ODS/M2A3 is also 7 apparently, which further eases this. The Platoon Leader and his RATELO are also supposed to ride in one of the tracks (with the Platoon Leader as the track commander). The Platoon Sergeant is also a track commander.
Just remember that the paper organization is a "perfect world" organization. A good starting place for sure, but not necessarily what one might see in the field.
Here is the mounted organization presented in FM 7-7J dated 7 May 1993:
-BFV 1-
Crew:
1x Platoon Leader
1x Senior Gunner
1x Vehicle Driver
Team A, 1st Squad:
1x Bradley Gunner (swaps places with Platoon Leader on dismount)
1x Team Leader w/ M16A2
1x Automatic Rifleman w/ M249
1x Grenadier w/ M16A2 and M203
1x Anti-Armor Specialist w/ M16A2 (also designated to use the M136 or SU-36 if available)
1x Forward Observer w/ M16A2
-BFV 2-
Crew:
1x Master Sergeant
1x Senior Gunner
1x Vehicle Driver
Team B, 1st Squad:
1x Squad Leader w/ M16A2
1x Team Leader w/ M16A2
2x Automatic Rifleman w/ M249
1x Anti-Armor Specialist w/ M16A2 (also designated to use the M136 or SU-36 if available)
1x Platoon RATELO w/ M16A2
-BFV 3-
Crew:
1x Staff Sergeant
1x Senior Gunner
1x Vehicle Driver
Team B, 2nd Squad:
1x Squad Leader w/ M16A2
1x Team Leader w/ M16A2
2x Automatic Rifleman w/ M249
1x Anti-Armor Specialist w/ M16A2 (also designated to use the M136 or SU-36 if available)
-BFV 4-
Crew:
1x Platoon Sergeant
1x Senior Gunner
1x Vehicle Driver
Team A, 2nd Squad:
1x Team Leader w/ M16A2
1x Automatic Rifleman w/ M249
1x Grenadier w/ M16A2 and M203
1x Anti-Armor Specialist w/ M16A2 (also designated to use the M136 or SU-36 if available)
1x Aidman
1x Forward Observer/RATELO
The dismounted element would consist of 2 squads of 9 men, plus the Platoon HQ.