I am using
BioEdit. It can perform an align of multiple sequences. Can this sequence similarity be outputted as a value, e.g. 70%?
No Bioedit is an alignment editor only. It is not in anyway an analysis program.
Dump the format in fasta or mega (its the option is available) and import it into Mega X here. You can quantify the diversity as a pairwise matrix, population analysis (if you have a clear a priori that you can subdivide your sequences into different groups) or phylogeny ... which provides diversity as mutations per alignment position. Its pointy and clicky and easy to use.