RobF has hit a very important point to consider.
A crank drive such as a BBS01 should allow a moderately fit person to hang with the faster groups, you can turn the speed limit up above 50 kph (can't remember the exact figure) so if your gearing is right keeping with the group should not be to difficult especially if fitted to a road bike or similar.
It depends upon how much help the op needs, a bit of assist or a lot?
A crank drive such as a BBS01 should allow a moderately fit person to hang with the faster groups, you can turn the speed limit up above 50 kph (can't remember the exact figure) so if your gearing is right keeping with the group should not be to difficult especially if fitted to a road bike or similar.
It depends upon how much help the op needs, a bit of assist or a lot?