Here have been involved should be used because the subject has More than one person involved,Which is plural , hence Verb used is also Plural (have) .
An eye examination is called for (1)/once a year to ensure that (2)/diabetes have not resulted in retinopathy, (3)/which could lead to blindness (4).
Most risk factors for (1)/pre-school overweight are modifiable (2)/and allows at-risk groups to (3)/be identified, say researchers (4).
The internal, or geothermal, (1)/heat from within the Earth are (2)/ responsible for the fact that (3)/temperature increases with depth (4).
The consecration of (1)/the Eucharist, as well as (2)/the administration, (3)/were performed in English (4).